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..676d92005870a89279973968d0f4f3e0bb3ef973 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);
+  // Invoked at the start of processing, before an EventTargeter is used to
+  // find the target of the event. If processing should not take place, marks
+  // |event| as handled. Otherwise updates |event| so that the targeter can
+  // operate correctly (e.g., it can be used to update the location of the
+  // event when dispatching from an EventSource in high-DPI) and updates any
+  // members in the event processor as necessary.
+  virtual void OnEventProcessingStarted(Event* event);
 
   // Invoked when the processing of |event| has finished (i.e., when no further
   // dispatching of |event| will be performed by this EventProcessor). Note