keyboard: Install a masked window-targeter in the keyboard container.

Install a masked window-targeter in the keyboard container so that event
targetting works correctly when used with EventProcessor for dispatching
events.

BUG=318879
R=ben@chromium.org

Review URL: https://codereview.chromium.org/139613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245048 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245048