ozone: Fix ozone/ecs build after r248154.

Update WindowTreeHostOzone to be an EventSource, and add the necessary
overrides.

BUG=318879
R=spang@chromium.org

Review URL: https://codereview.chromium.org/151653003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248243 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248243