Make composited scrolling codepaths co-operate.

Since we don’t have a functional layer-squashing solution
yet, it makes sense to leave in the logic for the stacking
container concept for the short term, and use it whenever
possible. If a layer cannot become a stacking container,
then we fall back to using the new codepath that’s being set
up in all the above patches.

This is blink-side patch 6 of https://docs.google.com/a/chromium.org/document/d/1Ln57_v6_4RmuuvtWnpsfBOUeDFrPsYNFW13dwuQ8UD8.

BUG=254111

Review URL: https://codereview.chromium.org/20103002

git-svn-id: svn://svn.chromium.org/blink/trunk@159168 bbb929c8-8fbe-4397-9dbb-9b2b20218538