aura: Root-windows should have valid FrameSinkId.

The root windows should use the FrameSinkId of the compositor. In regular
aura, compositors get valid FrameSinkId from context_factory_private().
If window server is hosting viz, then use the id from that.

BUG=754872

Change-Id: I7b1de89f9575ebd532042ffcd0b3a8058d85f251
Reviewed-on: https://chromium-review.googlesource.com/847333
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526735}

ui/aura/mus/window_tree_host_mus.cc

@@ -15,6 +15,7 @@
 #include "ui/aura/window_event_dispatcher.h"
 #include "ui/aura/window_tree_host_observer.h"
 #include "ui/base/class_property.h"
+#include "ui/base/ui_base_switches_util.h"
 #include "ui/display/display.h"
 #include "ui/display/screen.h"
 #include "ui/events/event.h"
@@ -56,7 +57,11 @@ WindowTreeHostMus::WindowTreeHostMus(WindowTreeHostMusInitParams init_params)
   // seems them at the time the window is created.
   for (auto& pair : init_params.properties)
     window_mus->SetPropertyFromServer(pair.first, &pair.second);
-  CreateCompositor(viz::FrameSinkId());
+  // If window-server is hosting viz, then use the FrameSinkId from the server.
+  // In other cases, let a valid FrameSinkId be selected by
+  // context_factory_private().
+  CreateCompositor(switches::IsMusHostingViz() ? window_mus->GetFrameSinkId()
+                                               : viz::FrameSinkId());
   if (!init_params.uses_real_accelerated_widget) {
     gfx::AcceleratedWidget accelerated_widget;
 // We need accelerated widget numbers to be different for each window and

ui/aura/window.cc

@@ -1091,6 +1091,12 @@ const viz::LocalSurfaceId& Window::GetLocalSurfaceId() const {
 }
 
 viz::FrameSinkId Window::GetFrameSinkId() const {
+  if (IsRootWindow()) {
+    DCHECK(host_);
+    auto* compositor = host_->compositor();
+    DCHECK(compositor);
+    return compositor->frame_sink_id();
+  }
   return port_->GetFrameSinkId();
 }
 

ui/aura/window_unittest.cc

@@ -3181,6 +3181,12 @@ TEST_P(WindowTest, WindowDestroyCompletesAnimations) {
   animator->RemoveObserver(&observer);
 }
 
+TEST_P(WindowTest, RootWindowUsesCompositorFrameSinkId) {
+  EXPECT_EQ(host()->compositor()->frame_sink_id(),
+            root_window()->GetFrameSinkId());
+  EXPECT_TRUE(root_window()->GetFrameSinkId().is_valid());
+}
+
 TEST_P(WindowTest, LocalSurfaceIdChanges) {
   Window window(nullptr);
   window.Init(ui::LAYER_NOT_DRAWN);