DevTools: add ids to compositor frames

This adds frameId to WebDevToolsAgent::didBeginFrame(), so we can
associate impl-side activity with the main thread frame.

BUG=305863
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/54273005

git-svn-id: svn://svn.chromium.org/blink/trunk@161147 bbb929c8-8fbe-4397-9dbb-9b2b20218538