Support buttons attribute for drag event (blink side).

Source/web/WebInputEventConversion.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 121 matching lines @@
     WebTouchEventBuilder(const Widget*, const LayoutObject*, const TouchEvent&);
 };
 
 // Converts GestureEvent to a corresponding WebGestureEvent.
 // NOTE: If event mapping fails, the type will be set to Undefined.
 class WebGestureEventBuilder : public WebGestureEvent {
 public:
     WebGestureEventBuilder(const Widget*, const LayoutObject*, const GestureEvent&);
 };
 
+unsigned toPlatformMouseEventModifiers(int webModifiers);
+
 } // namespace blink
 
 #endif