Crash in EventHandler::updateDragAndDrop

Crash in EventHandler::updateDragAndDrop
https://bugs.webkit.org/show_bug.cgi?id=77569

Reviewed by Alexey Proskuryakov.

Source/WebCore:

Test: fast/events/remove-target-with-shadow-in-drag.html

* page/EventHandler.cpp:
(WebCore::EventHandler::updateDragAndDrop):

LayoutTests:

* fast/events/remove-target-with-shadow-in-drag-expected.txt: Added.
* fast/events/remove-target-with-shadow-in-drag.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106488 268f45cc-cd09-0410-ab3c-d52691b4dbfc