Various cleanups to compositing code

There's no reason to call a functiont that only ASSERTs in release builds. Also,
clean up some minor style issues.

Review URL: https://codereview.chromium.org/316513002

git-svn-id: svn://svn.chromium.org/blink/trunk@175419 bbb929c8-8fbe-4397-9dbb-9b2b20218538