Correct the (x, y) position of dragstart event initiated by touch event

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 9a339bd89785e8814b5d0b6361fd02a41df373d4..3c2eabd3aff6732525fe098936b2173a6b22600f 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -2410,6 +2410,7 @@ bool EventHandler::handleGestureLongPress(const PlatformGestureEvent& gestureEve
         m_didStartDrag = false;
         m_mouseDownMayStartDrag = true;
         dragState().m_dragSrc = 0;
+        m_mouseDown = mouseDragEvent;

[varunjain, 2013/05/17 16:43:51]

I think this change is correct. But I am a little worried that we are assigning
a drag event to m_mouseDown. I think the field should be re-named to
m_dragSrcReferenceMouseEvent or something like that, because thats basically
what it is being used for, i.e. as a reference to construct the drag start event
sent in EventHandler::handleDrag()

[Hongbo Min, 2013/05/20 01:47:15]

I don't rename m_mouseDown field since it just records the mouse down event
triggered by pressing mouse button or long press gesture. You may notice the
m_mouseDown field is assigned in handleMousePressEvent and referenced by
handleDrag to dispatch drag event. 

Instead, I create a specific PlatformMouseEvent with MousePressed event type and
assign it m_mouseDown, by this way, it could avoid the potential side effect
caused by incorrect event type defined in mouseDragEvent for m_mouseDown field.

Does it make more sense now?

         m_mouseDownPos = m_frame->view()->windowToContents(mouseDragEvent.position());
         handleDrag(mev, DontCheckDragHysteresis);
         if (m_didStartDrag) {