Reland "gpu: Sync to primary monitor's vblank (windows)"

This reverts commit d332af51.

Fixed crash due to missing member var initialization.

TEST=ran failing gl_unittests locally

Original change's description:
> Revert "gpu: Sync to primary monitor's vblank (windows)"
>
> This reverts commit bd0cae50.
>
> Reason for revert: Cause failures in DirectCompositionSurfaceTest.*
> Builder: https://ci.chromium.org/p/chromium/builders/ci/Win10%20Debug%20%28NVIDIA%29?limit=200
> First build: https://ci.chromium.org/p/chromium/builders/ci/Win10%20Debug%20%28NVIDIA%29/22377?blamelist=1#overview-tab
>
> Crash example:
> ...snip...
>
> Original change's description:
> > gpu: Sync to primary monitor's vblank (windows)
> >
> > DWM composition is limited to primary monitor's refresh rate.  Prior to
> > GPU vsync we would use DWM composition rate to drive begin frames.
> >
> > With GPU vsync we started using per monitor vblank to limit begin frames
> > in case primary monitor had higher refresh rate, but this might be
> > causing draw duration and omnibar latency regressions.  MS platform team
> > also suggested syncing to primary monitor only.  In any case the correct
> > approach would be to further limit begin frames based on event queries
> > from 2 frames back.
> >
> > This CL makes the GPU vsync begin frames sync to primary monitor, and
> > also makes the vsync thread a singleton since it doesn't have per-window
> > state any more.
> >
> > Bug: 953970
> > Change-Id: Id3d3f043cb847172b1d6ba4bd38b087ceb2d8631
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1711215
> > Reviewed-by: Zhenyao Mo <zmo@chromium.org>
> > Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#679832}
>
> TBR=zmo@chromium.org,sunnyps@chromium.org
>
> Change-Id: If4a762325952dd33283bf41a13321e759de51025
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 953970
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713510
> Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
> Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#679915}

TBR=zmo@chromium.org,sunnyps@chromium.org,kolos@chromium.org

Change-Id: I743026c218932916aba9d9c7722d8343e9fdbdd9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 953970
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716211
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680276}

ui/gl/BUILD.gn

@@ -307,6 +307,7 @@ jumbo_component("gl") {
       "gl_wgl_api_implementation.h",
       "swap_chain_presenter.cc",
       "swap_chain_presenter.h",
+      "vsync_observer.h",
       "vsync_provider_win.cc",
       "vsync_provider_win.h",
       "vsync_thread_win.cc",

ui/gl/direct_composition_child_surface_win.cc

@@ -107,7 +107,9 @@ bool DirectCompositionChildSurfaceWin::UseSwapChainFrameStatistics() {
   return base::FeatureList::IsEnabled(features::kSwapChainFrameStatistics);
 }
 
-DirectCompositionChildSurfaceWin::DirectCompositionChildSurfaceWin() {}
+DirectCompositionChildSurfaceWin::DirectCompositionChildSurfaceWin(
+    gfx::VSyncProvider* vsync_provider)
+    : vsync_provider_(vsync_provider) {}
 
 DirectCompositionChildSurfaceWin::~DirectCompositionChildSurfaceWin() {
   Destroy();
@@ -258,6 +260,8 @@ gfx::SwapResult DirectCompositionChildSurfaceWin::SwapBuffers(
   bool succeeded = ReleaseDrawTexture(false /* will_discard */);
 
   if (UseSwapChainFrameStatistics()) {
+    vsync_provider_->GetVSyncParametersIfAvailable(&last_vsync_time_,
+                                                   &last_vsync_interval_);
     CheckPendingFrames();
     // Enqueue callback after retiring previous callbacks so that it's called
     // after SwapBuffers() returns.
@@ -503,17 +507,6 @@ bool DirectCompositionChildSurfaceWin::SetEnableDCLayers(bool enable) {
   return true;
 }
 
-void DirectCompositionChildSurfaceWin::UpdateVSyncParameters(
-    base::TimeTicks vsync_time,
-    base::TimeDelta vsync_interval) {
-  last_vsync_time_ = vsync_time;
-  last_vsync_interval_ = vsync_interval;
-}
-
-bool DirectCompositionChildSurfaceWin::HasPendingFrames() const {
-  return !pending_frames_.empty();
-}
-
 void DirectCompositionChildSurfaceWin::CheckPendingFrames() {
   DCHECK(UseSwapChainFrameStatistics());
 

ui/gl/direct_composition_child_surface_win.h

@@ -12,11 +12,15 @@
 
 #include "ui/gl/gl_surface_egl.h"
 
+namespace gfx {
+class VSyncProvider;
+}  // namespace gfx
+
 namespace gl {
 
 class DirectCompositionChildSurfaceWin : public GLSurfaceEGL {
  public:
-  DirectCompositionChildSurfaceWin();
+  explicit DirectCompositionChildSurfaceWin(gfx::VSyncProvider* vsync_provider);
 
   static bool UseSwapChainFrameStatistics();
 
@@ -40,11 +44,6 @@ class DirectCompositionChildSurfaceWin : public GLSurfaceEGL {
               bool has_alpha) override;
   bool SetEnableDCLayers(bool enable) override;
 
-  void UpdateVSyncParameters(base::TimeTicks vsync_time,
-                             base::TimeDelta vsync_interval);
-  bool HasPendingFrames() const;
-  void CheckPendingFrames();
-
   const Microsoft::WRL::ComPtr<IDCompositionSurface>& dcomp_surface() const {
     return dcomp_surface_;
   }
@@ -76,6 +75,7 @@ class DirectCompositionChildSurfaceWin : public GLSurfaceEGL {
     // Presentation callback enqueued in SwapBuffers().
     PresentationCallback callback;
   };
+  void CheckPendingFrames();
   void EnqueuePendingFrame(PresentationCallback callback);
   void ClearPendingFrames();
 
@@ -109,6 +109,7 @@ class DirectCompositionChildSurfaceWin : public GLSurfaceEGL {
   // be called on the device.
   uint64_t dcomp_surface_serial_ = 0;
 
+  gfx::VSyncProvider* const vsync_provider_;
   base::TimeTicks last_vsync_time_;
   base::TimeDelta last_vsync_interval_ = base::TimeDelta::FromSecondsD(1. / 60);
 

ui/gl/direct_composition_surface_win.cc

@@ -172,15 +172,14 @@ DirectCompositionSurfaceWin::DirectCompositionSurfaceWin(
     : GLSurfaceEGL(),
       child_window_(parent_window),
       task_runner_(base::ThreadTaskRunnerHandle::Get()),
-      root_surface_(new DirectCompositionChildSurfaceWin()),
+      root_surface_(new DirectCompositionChildSurfaceWin(vsync_provider.get())),
       layer_tree_(std::make_unique<DCLayerTree>(
           settings.disable_nv12_dynamic_textures,
           settings.disable_larger_than_screen_overlays)),
-      vsync_provider_(std::move(vsync_provider)),
-      vsync_callback_(std::move(vsync_callback)),
       presentation_helper_(
-          std::make_unique<GLSurfacePresentationHelper>(vsync_provider_.get())),
-      weak_ptr_factory_(this) {}
+          std::make_unique<GLSurfacePresentationHelper>(vsync_provider.get())),
+      vsync_provider_(std::move(vsync_provider)),
+      vsync_callback_(std::move(vsync_callback)) {}
 
 DirectCompositionSurfaceWin::~DirectCompositionSurfaceWin() {
   Destroy();
@@ -396,27 +395,17 @@ bool DirectCompositionSurfaceWin::Initialize(GLSurfaceFormat format) {
   if (!root_surface_->Initialize(GLSurfaceFormat()))
     return false;
 
-  if (root_surface_->UseSwapChainFrameStatistics()) {
-    // Save weak ptr on main thread before any use on vsync thread.
-    main_thread_vsync_callback_ = base::BindRepeating(
-        &DirectCompositionSurfaceWin::HandleVSyncOnMainThread,
-        weak_ptr_factory_.GetWeakPtr());
-  }
-
-  if ((SupportsGpuVSync() && vsync_callback_) || main_thread_vsync_callback_) {
-    vsync_thread_ = std::make_unique<VSyncThreadWin>(
-        window_, d3d11_device_,
-        base::BindRepeating(
-            &DirectCompositionSurfaceWin::HandleVSyncOnVSyncThread,
-            base::Unretained(this)));
-  }
+  if (SupportsGpuVSync() && vsync_callback_)
+    vsync_thread_ = VSyncThreadWin::GetInstance();
 
   return true;
 }
 
 void DirectCompositionSurfaceWin::Destroy() {
-  // Destroy vsync thread because joining it could issue callbacks.
+  if (vsync_thread_) {
+    vsync_thread_->RemoveObserver(this);
     vsync_thread_ = nullptr;
+  }
   // Destroy presentation helper first because its dtor calls GetHandle.
   presentation_helper_ = nullptr;
   root_surface_->Destroy();
@@ -459,14 +448,11 @@ gfx::SwapResult DirectCompositionSurfaceWin::SwapBuffers(
     callback.Reset();
   }
 
-  gfx::SwapResult swap_result = root_surface_->SwapBuffers(std::move(callback));
-
-  if (swap_result == gfx::SwapResult::SWAP_ACK &&
+  gfx::SwapResult swap_result;
+  if (root_surface_->SwapBuffers(std::move(callback)) ==
+          gfx::SwapResult::SWAP_ACK &&
       layer_tree_->CommitAndClearPendingOverlays(root_surface_.get())) {
-    if (vsync_thread_) {
-      vsync_thread_->SetEnabled(root_surface_->HasPendingFrames() ||
-                                vsync_callback_enabled_);
-    }
+    swap_result = gfx::SwapResult::SWAP_ACK;
   } else {
     swap_result = gfx::SwapResult::SWAP_FAILED;
   }
@@ -547,34 +533,17 @@ bool DirectCompositionSurfaceWin::SupportsGpuVSync() const {
 
 void DirectCompositionSurfaceWin::SetGpuVSyncEnabled(bool enabled) {
   DCHECK(vsync_thread_);
-  if (vsync_callback_enabled_ == enabled)
-    return;
-  vsync_callback_enabled_ = enabled;
-  vsync_thread_->SetEnabled(root_surface_->HasPendingFrames() ||
-                            vsync_callback_enabled_);
-}
-
-void DirectCompositionSurfaceWin::HandleVSyncOnVSyncThread(
-    base::TimeTicks vsync_time,
-    base::TimeDelta vsync_interval) {
-  if (vsync_callback_)
-    vsync_callback_.Run(vsync_time, vsync_interval);
-
-  if (main_thread_vsync_callback_) {
-    task_runner_->PostTask(
-        FROM_HERE, base::BindOnce(main_thread_vsync_callback_, vsync_time,
-                                  vsync_interval));
+  if (enabled) {
+    vsync_thread_->AddObserver(this);
+  } else {
+    vsync_thread_->RemoveObserver(this);
   }
 }
 
-void DirectCompositionSurfaceWin::HandleVSyncOnMainThread(
-    base::TimeTicks vsync_time,
-    base::TimeDelta vsync_interval) {
-  // Check pending frames in root surface in case client stops issuing swaps.
-  root_surface_->UpdateVSyncParameters(vsync_time, vsync_interval);
-  root_surface_->CheckPendingFrames();
-  vsync_thread_->SetEnabled(root_surface_->HasPendingFrames() ||
-                            vsync_callback_enabled_);
+void DirectCompositionSurfaceWin::OnVSync(base::TimeTicks vsync_time,
+                                          base::TimeDelta interval) {
+  DCHECK(vsync_callback_);
+  vsync_callback_.Run(vsync_time, interval);
 }
 
 scoped_refptr<base::TaskRunner>

ui/gl/direct_composition_surface_win.h

@@ -16,6 +16,7 @@
 #include "ui/gl/child_window_win.h"
 #include "ui/gl/gl_export.h"
 #include "ui/gl/gl_surface_egl.h"
+#include "ui/gl/vsync_observer.h"
 
 namespace gl {
 class DCLayerTree;
@@ -23,7 +24,8 @@ class DirectCompositionChildSurfaceWin;
 class GLSurfacePresentationHelper;
 class VSyncThreadWin;
 
-class GL_EXPORT DirectCompositionSurfaceWin : public GLSurfaceEGL {
+class GL_EXPORT DirectCompositionSurfaceWin : public GLSurfaceEGL,
+                                              public VSyncObserver {
  public:
   using VSyncCallback =
       base::RepeatingCallback<void(base::TimeTicks, base::TimeDelta)>;
@@ -105,7 +107,6 @@ class GL_EXPORT DirectCompositionSurfaceWin : public GLSurfaceEGL {
   gfx::Vector2d GetDrawOffset() const override;
   bool SupportsGpuVSync() const override;
   void SetGpuVSyncEnabled(bool enabled) override;
-
   // This schedules an overlay plane to be displayed on the next SwapBuffers
   // or PostSubBuffer call. Overlay planes must be scheduled before every swap
   // to remain in the layer tree. This surface's backbuffer doesn't have to be
@@ -113,6 +114,9 @@ class GL_EXPORT DirectCompositionSurfaceWin : public GLSurfaceEGL {
   // tree at z-order 0.
   bool ScheduleDCLayer(const ui::DCRendererLayerParams& params) override;
 
+  // VSyncObserver implementation.
+  void OnVSync(base::TimeTicks vsync_time, base::TimeDelta interval) override;
+
   HWND window() const { return window_; }
 
   scoped_refptr<base::TaskRunner> GetWindowTaskRunnerForTesting();
@@ -127,33 +131,21 @@ class GL_EXPORT DirectCompositionSurfaceWin : public GLSurfaceEGL {
   ~DirectCompositionSurfaceWin() override;
 
  private:
-  void HandleVSyncOnVSyncThread(base::TimeTicks vsync_time,
-                                base::TimeDelta vsync_interval);
-
-  void HandleVSyncOnMainThread(base::TimeTicks vsync_time,
-                               base::TimeDelta vsync_interval);
-
   HWND window_ = nullptr;
   ChildWindowWin child_window_;
   scoped_refptr<base::SequencedTaskRunner> task_runner_;
 
   scoped_refptr<DirectCompositionChildSurfaceWin> root_surface_;
   std::unique_ptr<DCLayerTree> layer_tree_;
+  std::unique_ptr<GLSurfacePresentationHelper> presentation_helper_;
 
-  std::unique_ptr<VSyncThreadWin> vsync_thread_;
   std::unique_ptr<gfx::VSyncProvider> vsync_provider_;
-
   const VSyncCallback vsync_callback_;
-  bool vsync_callback_enabled_ = false;
-
-  std::unique_ptr<GLSurfacePresentationHelper> presentation_helper_;
+  VSyncThreadWin* vsync_thread_ = nullptr;
 
   Microsoft::WRL::ComPtr<ID3D11Device> d3d11_device_;
   Microsoft::WRL::ComPtr<IDCompositionDevice2> dcomp_device_;
 
-  VSyncCallback main_thread_vsync_callback_;
-  base::WeakPtrFactory<DirectCompositionSurfaceWin> weak_ptr_factory_;
-
   DISALLOW_COPY_AND_ASSIGN(DirectCompositionSurfaceWin);
 };
 

ui/gl/vsync_observer.h

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UI_GL_VSYNC_OBSERVER_H_
+#define UI_GL_VSYNC_OBSERVER_H_
+
+namespace gl {
+class GL_EXPORT VSyncObserver {
+ public:
+  // Called on vsync thread.
+  virtual void OnVSync(base::TimeTicks vsync_time,
+                       base::TimeDelta interval) = 0;
+
+ protected:
+  virtual ~VSyncObserver() {}
+};
+}  // namespace gl
+
+#endif  // UI_GL_VSYNC_OBSERVER_H_

ui/gl/vsync_thread_win.cc

@@ -5,6 +5,10 @@
 #include "ui/gl/vsync_thread_win.h"
 
 #include "base/bind.h"
+#include "base/memory/singleton.h"
+#include "base/stl_util.h"
+#include "ui/gl/gl_angle_util_win.h"
+#include "ui/gl/vsync_observer.h"
 
 namespace gl {
 namespace {
@@ -43,45 +47,51 @@ Microsoft::WRL::ComPtr<IDXGIOutput> DXGIOutputFromMonitor(
 }
 }  // namespace
 
-VSyncThreadWin::VSyncThreadWin(
-    HWND window,
-    Microsoft::WRL::ComPtr<ID3D11Device> d3d11_device,
-    VSyncCallback callback)
+// static
+VSyncThreadWin* VSyncThreadWin::GetInstance() {
+  return base::Singleton<VSyncThreadWin>::get();
+}
+
+VSyncThreadWin::VSyncThreadWin()
     : vsync_thread_("GpuVSyncThread"),
-      window_(window),
-      d3d11_device_(std::move(d3d11_device)),
-      callback_(std::move(callback)) {
-  DCHECK(window_);
-  DCHECK(callback_);
+      d3d11_device_(QueryD3D11DeviceObjectFromANGLE()) {
+  DCHECK(d3d11_device_);
   base::Thread::Options options;
-  // Inherit priority from GPU main thread which depends on finch flags.
-  options.priority = base::PlatformThread::GetCurrentThreadPriority();
+  options.priority = base::ThreadPriority::DISPLAY;
   vsync_thread_.StartWithOptions(std::move(options));
 }
 
 VSyncThreadWin::~VSyncThreadWin() {
-  SetEnabled(false);
+  {
+    base::AutoLock auto_lock(lock_);
+    observers_.clear();
+  }
   vsync_thread_.Stop();
 }
 
-void VSyncThreadWin::SetEnabled(bool enabled) {
+void VSyncThreadWin::AddObserver(VSyncObserver* obs) {
   base::AutoLock auto_lock(lock_);
-  if (enabled_ == enabled)
-    return;
-  enabled_ = enabled;
-  if (enabled_ && !started_) {
-    started_ = true;
+  observers_.insert(obs);
+  if (is_idle_) {
+    is_idle_ = false;
     vsync_thread_.task_runner()->PostTask(
         FROM_HERE,
         base::BindOnce(&VSyncThreadWin::WaitForVSync, base::Unretained(this)));
   }
 }
 
+void VSyncThreadWin::RemoveObserver(VSyncObserver* obs) {
+  base::AutoLock auto_lock(lock_);
+  observers_.erase(obs);
+}
+
 void VSyncThreadWin::WaitForVSync() {
-  HMONITOR monitor = MonitorFromWindow(window_, MONITOR_DEFAULTTONEAREST);
-  if (window_monitor_ != monitor) {
-    window_monitor_ = monitor;
-    window_output_ = DXGIOutputFromMonitor(monitor, d3d11_device_);
+  // From Raymond Chen's blog "How do I get a handle to the primary monitor?"
+  // https://devblogs.microsoft.com/oldnewthing/20141106-00/?p=43683
+  HMONITOR monitor = MonitorFromWindow(nullptr, MONITOR_DEFAULTTOPRIMARY);
+  if (primary_monitor_ != monitor) {
+    primary_monitor_ = monitor;
+    primary_output_ = DXGIOutputFromMonitor(monitor, d3d11_device_);
   }
 
   base::TimeDelta interval = base::TimeDelta::FromSecondsD(1.0 / 60);
@@ -102,13 +112,13 @@ void VSyncThreadWin::WaitForVSync() {
 
   base::TimeTicks wait_for_vblank_start_time = base::TimeTicks::Now();
   bool wait_for_vblank_succeeded =
-      window_output_ && SUCCEEDED(window_output_->WaitForVBlank());
+      primary_output_ && SUCCEEDED(primary_output_->WaitForVBlank());
 
   // WaitForVBlank returns very early instead of waiting until vblank when the
   // monitor goes to sleep.  We use 1ms as a threshold for the duration of
   // WaitForVBlank and fallback to Sleep() if it returns before that.  This
   // could happen during normal operation for the first call after the vsync
-  // callback is enabled, but it shouldn't happen often.
+  // thread becomes non-idle, but it shouldn't happen often.
   const auto kVBlankIntervalThreshold = base::TimeDelta::FromMilliseconds(1);
   base::TimeDelta wait_for_vblank_elapsed_time =
       base::TimeTicks::Now() - wait_for_vblank_start_time;
@@ -119,17 +129,15 @@ void VSyncThreadWin::WaitForVSync() {
   }
 
   base::AutoLock auto_lock(lock_);
-  DCHECK(started_);
-  if (enabled_) {
+  if (!observers_.empty()) {
     vsync_thread_.task_runner()->PostTask(
         FROM_HERE,
         base::BindOnce(&VSyncThreadWin::WaitForVSync, base::Unretained(this)));
-    // Release lock before running callback to guard against any reentracny
-    // deadlock.
-    base::AutoUnlock auto_unlock(lock_);
-    callback_.Run(base::TimeTicks::Now(), interval);
+    base::TimeTicks vsync_time = base::TimeTicks::Now();
+    for (auto* obs : observers_)
+      obs->OnVSync(vsync_time, interval);
   } else {
-    started_ = false;
+    is_idle_ = true;
   }
 }
 

ui/gl/vsync_thread_win.h

@@ -9,48 +9,53 @@
 #include <windows.h>
 #include <wrl/client.h>
 
+#include "base/containers/flat_set.h"
 #include "base/threading/thread.h"
 #include "ui/gl/gl_export.h"
 
-namespace gl {
+namespace base {
+template <typename T>
+struct DefaultSingletonTraits;
+}  // namespace base
 
-// Helper class that manages a thread for calling IDXGIOutput::WaitForVBlank()
-// for the output corresponding to the given |window|, and runs |callback| on
-// the same thread.  The callback can be enabled or disabled via SetEnabled().
-// This is used by DirectCompositionSurfaceWin to plumb vsync signal back to the
-// display compositor's BeginFrameSource.
+namespace gl {
+class VSyncObserver;
+// Helper singleton that wraps a thread for calling IDXGIOutput::WaitForVBlank()
+// for the primary monitor, and notifies observers on the same thread. Observers
+// can be added or removed on the main thread, and the vsync thread goes to
+// sleep if there are no observers. This is used by DirectCompositionSurfaceWin
+// to plumb vsync signal back to the display compositor's BeginFrameSource.
 class GL_EXPORT VSyncThreadWin {
  public:
-  using VSyncCallback =
-      base::RepeatingCallback<void(base::TimeTicks, base::TimeDelta)>;
-  VSyncThreadWin(HWND window,
-                 Microsoft::WRL::ComPtr<ID3D11Device> d3d11_device,
-                 VSyncCallback callback);
-  ~VSyncThreadWin();
+  static VSyncThreadWin* GetInstance();
 
-  void SetEnabled(bool enabled);
+  // These methods are not rentrancy safe, and shouldn't be called inside
+  // VSyncObserver::OnVSync.  It's safe to assume that these can be called only
+  // from the main thread.
+  void AddObserver(VSyncObserver* obs);
+  void RemoveObserver(VSyncObserver* obs);
 
  private:
+  friend struct base::DefaultSingletonTraits<VSyncThreadWin>;
+
+  VSyncThreadWin();
+  ~VSyncThreadWin();
+
   void WaitForVSync();
 
   base::Thread vsync_thread_;
 
   // Used on vsync thread only after initialization.
-  const HWND window_;
   const Microsoft::WRL::ComPtr<ID3D11Device> d3d11_device_;
-  const VSyncCallback callback_;
-
-  // Used on vsync thread exclusively.
-  HMONITOR window_monitor_ = nullptr;
-  Microsoft::WRL::ComPtr<IDXGIOutput> window_output_;
+  HMONITOR primary_monitor_ = nullptr;
+  Microsoft::WRL::ComPtr<IDXGIOutput> primary_output_;
 
   base::Lock lock_;
-  bool GUARDED_BY(lock_) enabled_ = false;
-  bool GUARDED_BY(lock_) started_ = false;
+  bool GUARDED_BY(lock_) is_idle_ = true;
+  base::flat_set<VSyncObserver*> GUARDED_BY(lock_) observers_;
 
   DISALLOW_COPY_AND_ASSIGN(VSyncThreadWin);
 };
-
 }  // namespace gl
 
 #endif  // UI_GL_VSYNC_THREAD_WIN_H_