Route OnDragEvent through ViewAndroid tree

ui/android/view_client.h

 // Copyright 2017 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_ANDROID_VIEW_CLIENT_H_
 #define UI_ANDROID_VIEW_CLIENT_H_
 
 #include "ui/android/ui_android_export.h"
 
 namespace ui {
 
+class DragEventAndroid;
 class MotionEventAndroid;
 
 // Client interface used to forward events from Java to native views.
 // Calls are dispatched to its children along the hierarchy of ViewAndroid.
 // Use bool return type to stop propagating the call i.e. overriden method
 // should return true to indicate that the event was handled and stop
 // the processing.
 class UI_ANDROID_EXPORT ViewClient {
  public:
   virtual bool OnTouchEvent(const MotionEventAndroid& event,
                             bool for_touch_handle);
   virtual bool OnMouseEvent(const MotionEventAndroid& event);
   virtual bool OnMouseWheelEvent(const MotionEventAndroid& event);
+  virtual bool OnDragEvent(const DragEventAndroid& event);
   virtual void OnPhysicalBackingSizeChanged();
 };
 
 }  // namespace ui
 
 #endif  // UI_ANDROID_VIEW_CLIENT_H_