Refactor to not actually update composited layer mappings when called outside of RenderLayerCompositor,
and instead register dirty bits for a later update.

BUG=339892

Review URL: https://codereview.chromium.org/142973009

git-svn-id: svn://svn.chromium.org/blink/trunk@166872 bbb929c8-8fbe-4397-9dbb-9b2b20218538