Share code between ScrollingCoordinator and FrameView that were running the same
algorithm to find paint invalidation containers across frame boundaries.

Put the shared method on RenderLayer, where it makes more sense.

BUG=384520

Review URL: https://codereview.chromium.org/355843003

git-svn-id: svn://svn.chromium.org/blink/trunk@177326 bbb929c8-8fbe-4397-9dbb-9b2b20218538