Add OVERRIDE to ui::TreeModelObserver overridden methods.

BUG=None
TEST=None

R=sky@chromium.org

Review URL: http://codereview.chromium.org/7046093

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88827 0039d316-1c4b-4281-b951-d872f2087c98