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

Source/core/events/MouseEvent.h

Index: Source/core/events/MouseEvent.h
diff --git a/Source/core/events/MouseEvent.h b/Source/core/events/MouseEvent.h
index 13af31cb53a50b8f60c7c9341f7676b2b1a83ebb..1295faa4b02f884bc7f85d9bda074c0e36cde561 100644
--- a/Source/core/events/MouseEvent.h
+++ b/Source/core/events/MouseEvent.h
@@ -78,6 +78,9 @@ public:
 
     bool fromTouch() const { return m_syntheticEventType == PlatformMouseEvent::FromTouch; }
 
+    bool isSynthetic() const { return m_syntheticEventType == PlatformMouseEvent::Synthetic; }

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

Again this name is confusing. IIRC we recently replaced an 'isSynthetic'
accessor with the m_syntheticEventType system to reduce confusion over different
types of 'synthetic' events.  But now I see MouseEventDispatchMediator has it's
own notion of 'synthetic' that you're mirroring here.  To avoid confusion I
think we should unify the two mechanisms together.  What I'd suggest is this:

 - Remove MouseEventDispatchMediator::m_mouseEventType
 - Update the only place that sets this (Node::dispatchEvent) to instead call
MouseEvent::setFromScript() to reset the m_syntheticEventType to 'FromScript'.
 - For consistency, update the MouseEvent constructor / JS creation code to also
set the synthetic event type to 'FromScript'
 - The comment above the definition of 'FromScript' should make clear that the
event was either created or re-dispatched by JavaScript (we don't want script to
be able to catch a real MouseEvent and dispatch it later as if it was still
real).
 - Now you can just test 'isFromScript' in
MouseEventDispatchMediator::dispatchEvent and in your default handling logic.

+    void markAsSynthetic() { m_syntheticEventType = PlatformMouseEvent::Synthetic; }
+
     virtual const AtomicString& interfaceName() const override;
 
     virtual bool isMouseEvent() const override;