GTK: don't get stuck in render view drag.
GTK: don't get stuck in render view drag.

Everyone once in a while gtk_drag_begin() returns NULL meaning the drag failed to start. Inform webkit of this.

BUG=30580
TEST=see bug

Review URL: http://codereview.chromium.org/548108

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@36812 0039d316-1c4b-4281-b951-d872f2087c98

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