Rename WMEventType to match the style.

BUG=319325
R=abodenha@chromium.org

Review URL: https://codereview.chromium.org/186653003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254815 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254815