aura: Start using EventPrcessor interface for event dispatch.

ui/events/event_processor.h

 // Copyright 2013 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.
 
 #ifndef UI_EVENTS_EVENT_PROCESSOR_H_
 #define UI_EVENTS_EVENT_PROCESSOR_H_
 
 #include "ui/events/event_dispatcher.h"
 #include "ui/events/event_source.h"
 
 namespace ui {
 
 // EventProcessor receives an event from an EventSource and dispatches it to a
 // tree of EventTargets.
 class EVENTS_EXPORT EventProcessor : public EventDispatcherDelegate {
  public:
   virtual ~EventProcessor() {}
 
   // Returns the root of the tree this event processor owns.
   virtual EventTarget* GetRootTarget() = 0;
 
   // Dispatches an event received from the EventSource to the tree of
   // EventTargets (whose root is returned by GetRootTarget()).
   virtual EventDispatchDetails OnEventFromSource(Event* event)
       WARN_UNUSED_RESULT;
+
+ protected:
+  // Prepares the event so that it can be dispatched. This is invoked before
+  // an EventTargeter is used to find the target of the event. So this can be
+  // used to update the event so that the targeter can operate correctly (e.g.
+  // it can be used to updated the location of the event when disptaching from
+  // an EventSource in high-DPI).
+  virtual void PrepareEventForDispatch(Event* event);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_EVENT_PROCESSOR_H_