Support buttons attribute for drag event (blink side).

Source/web/WebInputEventConversion.cpp

 /*
  * 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 97 matching lines @@
         newModifiers |= PlatformEvent::ShiftKey;
     if (webModifiers & WebInputEvent::ControlKey)
         newModifiers |= PlatformEvent::CtrlKey;
     if (webModifiers & WebInputEvent::AltKey)
         newModifiers |= PlatformEvent::AltKey;
     if (webModifiers & WebInputEvent::MetaKey)
         newModifiers |= PlatformEvent::MetaKey;
     return newModifiers;
 }
 
-static unsigned toPlatformMouseEventModifiers(int webModifiers)
+unsigned toPlatformMouseEventModifiers(int webModifiers)
 {
     unsigned newModifiers = toPlatformEventModifiers(webModifiers);
     if (webModifiers & WebInputEvent::LeftButtonDown)
         newModifiers |= PlatformEvent::LeftButtonDown;
     if (webModifiers & WebInputEvent::MiddleButtonDown)
         newModifiers |= PlatformEvent::MiddleButtonDown;
     if (webModifiers & WebInputEvent::RightButtonDown)
         newModifiers |= PlatformEvent::RightButtonDown;
     return newModifiers;
 }
@@ skipping 681 matching lines @@
     modifiers = getWebInputModifiers(event);
 
     globalX = event.screenX();
     globalY = event.screenY();
     IntPoint localPoint = convertAbsoluteLocationForLayoutObject(event.absoluteLocation(), *layoutObject);
     x = localPoint.x();
     y = localPoint.y();
 }
 
 } // namespace blink