Remove early exits from updateCompositingLayersAfterStyleChange

Now that updateCompositingLayersAfterStyleChange just sets dirty bits, there's
no reason to early exit from this function if layout is pending.

The one exception is the one case where we still do more work than just set a
dirty bit, which is inside setNeedsCompositingUpdate. This CL pushes the one
check we still need closer to the reason its needed.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/266383002

git-svn-id: svn://svn.chromium.org/blink/trunk@173363 bbb929c8-8fbe-4397-9dbb-9b2b20218538