Gracefully handle drag cancellation during drag start.

Source/core/page/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * 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.
@@ skipping 3245 matching lines @@
             // The renderer has disappeared, this can happen if the onStartDrag handler has hidden
             // the element in some way. In this case we just kill the drag.
             return false;
         }
     }
 
     DragController& dragController = m_frame->page()->dragController();
     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.
     dragState().m_dragDataTransfer->setAccessPolicy(DataTransferImageWritable);
 
     if (m_mouseDownMayStartDrag) {
         // Dispatching the event could cause Page to go away. Make sure it's still valid before trying to use DragController.
         m_didStartDrag = m_frame->page() && dragController.startDrag(m_frame, dragState(), event.event(), m_mouseDownPos);
         // FIXME: This seems pretty useless now. The gesture code uses this as a signal for
         // whether or not the drag started, but perhaps it can simply use the return value from
@@ skipping 573 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace blink