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,
+        // Synthetic mouse events that do not fire the default action handler
+        Synthetic,

[Rick Byers, 2015/03/05 16:34:18]

Nit: can we call this 'FromScript' to distinguish it from some other types of
'synthetic' mouse events (eg. FromTouch and some special cases currently lumped
into RealOrIndistinguishable like the synthetic mousemove that occurs during
scrolling)?

     };
 
     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