Avoid two targeting phases in aura client-lib and EventProcessor.

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 {
 
+class EventTargeter;
+
 // EventProcessor receives an event from an EventSource and dispatches it to a
 // tree of EventTargets.
 class EVENTS_EXPORT EventProcessor : public EventDispatcherDelegate {
  public:
   ~EventProcessor() override {}
 
-  // Returns the root of the tree this event processor owns.
-  virtual EventTarget* GetRootTarget() = 0;
+  // Returns the EventTarget with the right EventTargeter that we should use for
+  // dispatching this |event|.
+  virtual EventTarget* GetRootForEvent(Event* event) = 0;
+
+  virtual EventTargeter* GetDefaultEventTargeter() = 0;

[sadrul, 2017/02/13 17:05:42]

Document, e.g. "If the root target returned by GetRootForEvent() does not have a
targeter set, then the default targeter is used to find the target."

[riajiang, 2017/02/13 23:30:34]

Done.

 
   // Dispatches an event received from the EventSource to the tree of
   // 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:
   // 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
   // that the last target to which |event| was dispatched may have been
   // destroyed.
   virtual void OnEventProcessingFinished(Event* event);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_EVENT_PROCESSOR_H_