Extract CompositingRequirementsUpdater from RenderLayerCompositor

This CL extracts the algorithm for computing the compositing requirements from
RenderLayerCompositor. This code has almost no interaction with the private
state in RenderLayerCompositor, which means its easy to separate. In fact, the
one interaction it does have with RenderLayerCompositor's private state is
wrong!

Review URL: https://codereview.chromium.org/227713007

git-svn-id: svn://svn.chromium.org/blink/trunk@171040 bbb929c8-8fbe-4397-9dbb-9b2b20218538