Make RenderLayer::compositedLayerMapping assert that compositing state is up to date.

compositedLayerMapping is not up to date unless we've updated
compositing state. This assert exposes a bunch of new chicken
and egg compositing bugs.

Review URL: https://codereview.chromium.org/173973003

git-svn-id: svn://svn.chromium.org/blink/trunk@167636 bbb929c8-8fbe-4397-9dbb-9b2b20218538