Do not set the last mouse location from synthesized event.
 The last mouse location should be the updated only by real event because it should point to the real mouse point and synthesized enter/leave may not contain the valid point.

minor: remove unnecessary cast.

BUG=None
TEST=TBD.

Review URL: https://codereview.chromium.org/143303006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251291 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251291