Prevent default actions for JS-generated mouse events other than click

Source/platform/PlatformMouseEvent.h

 /*
  * Copyright (C) 2004, 2005, 2006, 2009 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 23 matching lines @@
 // These button numbers match the ones used in the DOM API, 0 through 2, except for NoButton which isn't specified.
 enum MouseButton { NoButton = -1, LeftButton, MiddleButton, RightButton };
 
 class PlatformMouseEvent : public PlatformEvent {
 public:
     enum SyntheticEventType {
         // Real mouse input events or synthetic events that behave just like real events
         RealOrIndistinguishable,
         // Mouse events derived from touch input
         FromTouch,
+        // Mouse events created from script or dispatched from script
+        FromScript
     };
 
     PlatformMouseEvent()
         : PlatformEvent(PlatformEvent::MouseMoved)
         , m_button(NoButton)
         , m_clickCount(0)
         , m_synthesized(RealOrIndistinguishable)
     {
     }
 
@@ skipping 41 matching lines @@
     IntPoint m_globalPosition;
     IntPoint m_movementDelta;
     MouseButton m_button;
     int m_clickCount;
     SyntheticEventType m_synthesized;
 };
 
 } // namespace blink
 
 #endif // PlatformMouseEvent_h