Cleanup: Remove unused forward declarations from compositor.h

Plus remove a few ui:: prefix where it isn't necessary and two
unnecessary includes.

BUG=None
TEST=compositor_unittests
R=danakj@chromium.org

Review URL: https://codereview.chromium.org/112613002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240136 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240136