aura: Reland r266681 to debug WindowObserver crashes.

Add code to make the aura::WindowObserver interface crash
when it's destroyed without having unregistered its interest
in all aura::Windows that it was observing. This will
hopefully help track down the source of crashes that are
seen when closing video windows on Google Play.

https://codereview.chromium.org/303163003/ hopefully fixes
the test crashes that resulted in this change being reverted
earlier.

BUG=365364

Review URL: https://codereview.chromium.org/309573004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274086 0039d316-1c4b-4281-b951-d872f2087c98

ui/aura/aura.gyp

@@ -72,6 +72,7 @@
         'window_event_dispatcher.h',
         'window_delegate.h',
         'window_layer_type.h',
+        'window_observer.cc',
         'window_observer.h',
         'window_targeter.cc',
         'window_targeter.h',

ui/aura/window.cc

@@ -644,10 +644,12 @@ gfx::NativeCursor Window::GetCursor(const gfx::Point& point) const {
 }
 
 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_observer.cc

+// Copyright 2014 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.
+
+#include "ui/aura/window_observer.h"
+
+#include "base/logging.h"
+#include "ui/aura/window.h"
+
+namespace aura {
+
+WindowObserver::WindowObserver() : observing_(0) {
+}
+
+WindowObserver::~WindowObserver() {
+  // TODO(flackr): Remove this check and observing_ counter when the cause of
+  // http://crbug.com/365364 is discovered.
+  CHECK_EQ(0, observing_);
+}
+
+void WindowObserver::OnObservingWindow(aura::Window* window) {
+  if (!window->HasObserver(this))
+    observing_++;
+}
+
+void WindowObserver::OnUnobservingWindow(aura::Window* window) {
+  if (window->HasObserver(this))
+    observing_--;
+}
+
+}  // namespace aura

ui/aura/window_observer.h

@@ -31,6 +31,8 @@ class AURA_EXPORT WindowObserver {
     Window* receiver;   // The window receiving the notification.
   };
 
+  WindowObserver();
+
   // Called when a window is added or removed. Notifications are sent to the
   // following hierarchies in this order:
   // 1. |target|.
@@ -111,7 +113,22 @@ class AURA_EXPORT WindowObserver {
                                               Window* new_root) {}
 
  protected:
-  virtual ~WindowObserver() {}
+  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);
 };
 
 }  // namespace aura

ui/aura/window_unittest.cc

@@ -2229,6 +2229,7 @@ TEST_F(WindowTest, RootWindowSetWhenReparenting) {
   parent1.SetBounds(gfx::Rect(10, 10, 300, 300));
   parent2.SetBounds(gfx::Rect(20, 20, 300, 300));
 
+  BoundsChangedWindowObserver observer;
   Window child(NULL);
   child.Init(aura::WINDOW_LAYER_NOT_DRAWN);
   child.SetBounds(gfx::Rect(5, 5, 100, 100));
@@ -2242,7 +2243,6 @@ TEST_F(WindowTest, RootWindowSetWhenReparenting) {
   gfx::Rect new_bounds(gfx::Rect(35, 35, 50, 50));
   child.SetBounds(new_bounds);
 
-  BoundsChangedWindowObserver observer;
   child.AddObserver(&observer);
 
   // Reparenting the |child| will cause it to get moved. During this move