Convert aura::WindowObserver to use CheckedObserver

It had some logic for counting window under observation, but
CheckedObserver is a better way of catching use-after-free errors.

Bug: none
Test: bots
Change-Id: I21020c194a848c21e434ad0e8583684053b1f663
Reviewed-on: https://chromium-review.googlesource.com/c/1315991
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605092}

ui/aura/window.cc

@@ -550,12 +550,10 @@ bool Window::ShouldRestackTransientChildren() {
 }
 
 void Window::AddObserver(WindowObserver* observer) {
-  observer->OnObservingWindow(this);
   observers_.AddObserver(observer);
 }
 
 void Window::RemoveObserver(WindowObserver* observer) {
-  observer->OnUnobservingWindow(this);
   observers_.RemoveObserver(observer);
 }
 

ui/aura/window.h

@@ -659,7 +659,7 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
   // Makes the window pass all events through to any windows behind it.
   ws::mojom::EventTargetingPolicy event_targeting_policy_;
 
-  base::ObserverList<WindowObserver, true>::Unchecked observers_;
+  base::ObserverList<WindowObserver, true> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(Window);
 };

ui/aura/window_observer.cc

@@ -5,27 +5,13 @@
 #include "ui/aura/window_observer.h"
 
 #include "base/logging.h"
-#include "ui/aura/window.h"
 
 namespace aura {
 
-WindowObserver::WindowObserver() : observing_(0) {
-}
+WindowObserver::WindowObserver() = default;
 
 WindowObserver::~WindowObserver() {
-  CHECK_EQ(0, observing_);
-}
-
-void WindowObserver::OnObservingWindow(aura::Window* window) {
-  if (!window->HasObserver(this))
-    observing_++;
+  CHECK(!IsInObserverList());
 }
 
-void WindowObserver::OnUnobservingWindow(aura::Window* window) {
-  if (window->HasObserver(this))
-    observing_--;
-}
-
-void WindowObserver::OnEmbeddedAppDisconnected(Window* window) {}
-
 }  // namespace aura

ui/aura/window_observer.h

@@ -5,7 +5,7 @@
 #ifndef UI_AURA_WINDOW_OBSERVER_H_
 #define UI_AURA_WINDOW_OBSERVER_H_
 
-#include "base/macros.h"
+#include "base/observer_list_types.h"
 #include "base/strings/string16.h"
 #include "ui/aura/aura_export.h"
 #include "ui/compositor/property_change_reason.h"
@@ -19,7 +19,7 @@ namespace aura {
 
 class Window;
 
-class AURA_EXPORT WindowObserver {
+class AURA_EXPORT WindowObserver : public base::CheckedObserver {
  public:
   struct HierarchyChangeParams {
     enum HierarchyChangePhase {
@@ -172,25 +172,10 @@ class AURA_EXPORT WindowObserver {
 
   // Called when the app embedded in |window| disconnects (is no longer
   // embedded).
-  virtual void OnEmbeddedAppDisconnected(Window* window);
+  virtual void OnEmbeddedAppDisconnected(Window* window) {}
 
  protected:
-  virtual ~WindowObserver();
-
- private:
-  friend class Window;
-
-  // Called when this is added as an observer on |window|.
-  void OnObservingWindow(Window* window);
-
-  // Called when this is removed from the observers on |window|.
-  void OnUnobservingWindow(Window* window);
-
-  // Tracks the number of windows being observed to track down
-  // http://crbug.com/365364.
-  int observing_;
-
-  DISALLOW_COPY_AND_ASSIGN(WindowObserver);
+  ~WindowObserver() override;
 };
 
 }  // namespace aura

ui/aura/window_port.cc

@@ -16,7 +16,7 @@ WindowPort* WindowPort::Get(Window* window) {
 }
 
 // static
-base::ObserverList<WindowObserver, true>::Unchecked* WindowPort::GetObservers(
+base::ObserverList<WindowObserver, true>* WindowPort::GetObservers(
     Window* window) {
   return &(window->observers_);
 }

ui/aura/window_port.h

@@ -146,8 +146,7 @@ class AURA_EXPORT WindowPort {
   static WindowPort* Get(Window* window);
 
   // Returns the ObserverList of a Window.
-  static base::ObserverList<WindowObserver, true>::Unchecked* GetObservers(
-      Window* window);
+  static base::ObserverList<WindowObserver, true>* GetObservers(Window* window);
 
  private:
   const Type type_;