Fix some style nits in core/rendering/compositing

This CL shouldn't have any functional change. It's just fixing some style nits
I noticed while studying the code.

Review URL: https://codereview.chromium.org/218473004

git-svn-id: svn://svn.chromium.org/blink/trunk@170729 bbb929c8-8fbe-4397-9dbb-9b2b20218538