Revert "Revert "Make composited scrolling codepaths co-operate.""

This reverts commit c48a005859152716e9c731fc8c686e98d04fba69.

This was speculatively reverted, but it turned out not to be the culprit. Relanding.

R=hartmanng@chromium.org
BUG=306566

Review URL: https://codereview.chromium.org/26253006

git-svn-id: svn://svn.chromium.org/blink/trunk@159892 bbb929c8-8fbe-4397-9dbb-9b2b20218538