Promote extra layers to remove corner cases from overflow:scroll fast path.

Adds 2 new compositing reasons (both only active behind the
compositorDrivenAcceleratedScrollingEnabled Internals setting).
These handle compositing layers when they are the direct
child of a non-stacking context overflow-scroll div, and also
layers that are unclipped descendants of any overflow-scroll div.

This is blink-side patch 2 of https://docs.google.com/a/chromium.org/document/d/1Ln57_v6_4RmuuvtWnpsfBOUeDFrPsYNFW13dwuQ8UD8.

BUG=249357,249349

Review URL: https://chromiumcodereview.appspot.com/18938002

git-svn-id: svn://svn.chromium.org/blink/trunk@156486 bbb929c8-8fbe-4397-9dbb-9b2b20218538