Only update style-determined composited reasons during the chicken/egg compositing update

Prior to this CL, we updated both the style determined and the non-style
determined compositing reasons during the chicken/egg compositing update. Now
we only update the style-determined reasons, which means the non-style
determined reasons are only computed during the real compositing update.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/342593003

git-svn-id: svn://svn.chromium.org/blink/trunk@176374 bbb929c8-8fbe-4397-9dbb-9b2b20218538