Put style recalc optimizations behind a flag (try 2).

BUG=335923,335715
R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/132693013

git-svn-id: svn://svn.chromium.org/blink/trunk@165538 bbb929c8-8fbe-4397-9dbb-9b2b20218538