Factor CompositingLayerAssigner out of RenderLayerCompositor

This CL moves the algorithm for assigning layers to backings out of
RenderLayerCompositor and into an algorithm object named
CompositingLayerAssigner. Moving separating this code makes it easier to
understand how this algorithm works.

Review URL: https://codereview.chromium.org/269123002

git-svn-id: svn://svn.chromium.org/blink/trunk@173342 bbb929c8-8fbe-4397-9dbb-9b2b20218538