Introduce EventProcessor::OnEventProcessingStarted()

ui/events/event_processor.h

Index: ui/events/event_processor.h
diff --git a/ui/events/event_processor.h b/ui/events/event_processor.h
index b4e8f986e6e8adf6bef017a638895a6e823d1733..c1dfdfe1ce1f6f940783eaaf5765c15cc06084f0 100644
--- a/ui/events/event_processor.h
+++ b/ui/events/event_processor.h
@@ -23,16 +23,19 @@ class EVENTS_EXPORT EventProcessor : public EventDispatcherDelegate {
   // EventTargets (whose root is returned by GetRootTarget()).  The co-ordinate
   // space of the source must be the same as the root target, except that the
   // target may have a high-dpi scale applied.
+  // TODO(tdanderson|sadrul): This is only virtual for testing purposes. It
+  //                          should not be virtual at all.
   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);
+  // Returns true if the event should be processed and 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
+  // update the location of the event when disptaching from an EventSource in
+  // high-DPI).
+  virtual bool OnEventProcessingStarted(Event* event);

[sadrul, 2014/09/15 14:10:36]

I remember we briefly discussed making this not return a bool, and instead
marking the event as handled if the event should not be dispatched. Can you
remind me why that wouldn't work?

[tdanderson, 2014/09/15 18:20:26]

Sorry, I actually don't remember discussing that option. Let me give it a try
(I'll ping you when I've uploaded another CL). The first concern that comes to
mind is that by marking these events as handled we would prevent them from being
forwarded to the menu controller in MenuHostRootView.

 
   // Invoked when the processing of |event| has finished (i.e., when no further
   // dispatching of |event| will be performed by this EventProcessor). Note