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