Defer the real work in updateCompositingLayers until it's really needed.

Many types of change on a web page end up requiring that
compositing state gets updated. Currently this work is performed
immediately after layout, recalcStyle, and other entry points.

However, this work may happen surprisingly many times per actual
rendered frame, which is absolutely wasteful - nothing except the
most recent compositing state will matter when the frame finally
gets drawn. So, instead we can defer all compositing update work
until the compositor is ready to ask for updated compositing state.

BUG=229679,302726
R=jamesr@chromium.org, vollick@chromium.org

Review URL: https://codereview.chromium.org/26110004

git-svn-id: svn://svn.chromium.org/blink/trunk@159961 bbb929c8-8fbe-4397-9dbb-9b2b20218538