Add Document::scheduleRenderTreeUpdateIfNeeded

Instead of having a special early out inside markAncestorsWithChildNeedsStyleRecalc()
to avoid calling scheduleRenderTreeUpdate() we should just generalize that
optimization and use it in all places that are attempting to schedule
recalcs.

Review URL: https://codereview.chromium.org/223843006

git-svn-id: svn://svn.chromium.org/blink/trunk@170812 bbb929c8-8fbe-4397-9dbb-9b2b20218538