x11: Fix a memory leak for mouse events.

Make sure the event-cookie is destroyed properly when mouse-move eent
coalescing ends because a different kind of event is in the X queue.

BUG=317682
R=derat@chromium.org

Review URL: https://codereview.chromium.org/89513002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237529 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237529