aura: Reland r266681 to debug WindowObserver crashes.

ui/aura/window_observer.cc

Index: ui/aura/window_observer.cc
diff --git a/ui/aura/window_observer.cc b/ui/aura/window_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8f878267b2322fbb9b812c8c86041321e2f2017e
--- /dev/null
+++ b/ui/aura/window_observer.cc
@@ -0,0 +1,31 @@
+// Copyright (c) 2013 The Chromium Authors. All rights reserved.

[flackr, 2014/05/30 00:12:22]

nit: This copyright notice is out of date now :-).

+// 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