Add GPURasterizationTrigger

Instead of reading from the settings object repeatedly when finding direct
compositing reasons, most settings are cached in the compositing triggers bit
vector. This CL moves the GPU rasterization trigger to the same model.

Review URL: https://codereview.chromium.org/206543005

git-svn-id: svn://svn.chromium.org/blink/trunk@169705 bbb929c8-8fbe-4397-9dbb-9b2b20218538