events: Introduce PlatformEventDispatcher and PlatformEventSource.

ui/events/platform/scoped_event_dispatcher.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/events/platform/scoped_event_dispatcher.h"
+
+#include "ui/events/platform/platform_event_source.h"
+
+namespace ui {
+
+ScopedEventDispatcher::ScopedEventDispatcher(
+    PlatformEventDispatcher** scoped_dispatcher,
+    PlatformEventDispatcher* new_dispatcher)
+    : original_(*scoped_dispatcher),
+      restore_(scoped_dispatcher, new_dispatcher) {}
+
+ScopedEventDispatcher::~ScopedEventDispatcher() {
+  PlatformEventSource::GetInstance()->OnOverriddenDispatcherRestored();
+}
+
+}  // namespace ui