Remove compositing trigger for CSS transitions

This feature isn't enabled in any shipping configuration. We might want to add
it back after shipping layer squashing.

BUG=178119

Review URL: https://codereview.chromium.org/206593005

git-svn-id: svn://svn.chromium.org/blink/trunk@169704 bbb929c8-8fbe-4397-9dbb-9b2b20218538