Move compositing code into core/rendering/compositing

This CL moves the compositing-related code from core/rendering into a
compositing subdirectory. These file are quite large an complicated. After
moving them to a directory, the next step is to break down the code into
smaller, easier-to-understand pieces.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/178353003

git-svn-id: svn://svn.chromium.org/blink/trunk@167697 bbb929c8-8fbe-4397-9dbb-9b2b20218538