Remove CompositingChickenEgg issue in ScrollingCoordinator

Instead of eagerly calling WebLayer::setShouldScrollOnMainThread during style
recalc (which requires knowledge of compositing information), we defer the work
until the next compositing update.

R=vollick@chromium.org
BUG=339846

Review URL: https://codereview.chromium.org/158923004

git-svn-id: svn://svn.chromium.org/blink/trunk@166886 bbb929c8-8fbe-4397-9dbb-9b2b20218538