Teach CompositingUpdateOnCompositedScroll about partial compositing updates

This CL teaches the codepath that updates the compositing tree after a
composited scroll how to do a partial update rather than spamming all the
CompositedLayerMappings.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/266663002

git-svn-id: svn://svn.chromium.org/blink/trunk@173050 bbb929c8-8fbe-4397-9dbb-9b2b20218538