Directly update opacity

This CL expands the codepath that directly updates transforms to directly
update opacity. We don't need to do anything clever for opacity because having
opacity doesn't affect the early stages of the compositing pipeline.

I've also added some comments explaining why we bail out from a direct update
in a couple places.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/372433002

git-svn-id: svn://svn.chromium.org/blink/trunk@177595 bbb929c8-8fbe-4397-9dbb-9b2b20218538