Compute potential subtree reasons for compositing during style update

Prior to this CL, we recomputed the subtree reasons for compositing for every
RenderLayer in the entire document during every compositing update. After this
CL, we compute this information for each RenderLayer only when its style changes.
That means we now do work proportional to the change in a given frame rather than
proportional to the entire document.

Review URL: https://codereview.chromium.org/339503005

git-svn-id: svn://svn.chromium.org/blink/trunk@176580 bbb929c8-8fbe-4397-9dbb-9b2b20218538