Gracefully handle drag cancellation during drag start.

Source/core/page/EventHandler.cpp

Index: Source/core/page/EventHandler.cpp
diff --git a/Source/core/page/EventHandler.cpp b/Source/core/page/EventHandler.cpp
index b2e91147ce635d4132727b17ed825571bf86d053..6838c622de220b457b1683bc9c5ccae68bf96d9d 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -3263,7 +3263,7 @@ bool EventHandler::tryStartDrag(const MouseEventWithHitTestResults& event)
     if (!dragController.populateDragDataTransfer(m_frame, dragState(), m_mouseDownPos))
         return false;
     m_mouseDownMayStartDrag = dispatchDragSrcEvent(EventTypeNames::dragstart, m_mouseDown)
-        && !m_frame->selection().isInPasswordField();
+        && !m_frame->selection().isInPasswordField() && dragState().m_dragSrc;

[dcheng, 2014/08/20 10:56:24]

I wonder if it's worth adding a comment about why we need to check if there's
still a drag source after dispatching the event. The drag code is already fairly
unintuitive as it is, and this can't "normally" happen.

[sof, 2014/08/20 11:06:42]

Good idea, added one - does it clarify ?

 
     // Invalidate clipboard here against anymore pasteboard writing for security. The drag
     // image can still be changed as we drag, but not the pasteboard data.