Whittle down some compositing chicken/egg hacks

These chicken/egg hacks appear to only be needed for scroll parts. This CL
moves them into RenderLayerScrollableArea so that we don't need to apply them
constantly for RenderLayers that aren't scrollable.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/220473002

git-svn-id: svn://svn.chromium.org/blink/trunk@170522 bbb929c8-8fbe-4397-9dbb-9b2b20218538