Remove compositing reasons from the public Web API

Now that compositing reasons are being plumbed via GraphicsLayerDebugInfo, and
cc is no longer using its side of the plumbing, it's time to remove the blink
side as well.

BUG=None

Review URL: https://codereview.chromium.org/131363005

git-svn-id: svn://svn.chromium.org/blink/trunk@164890 bbb929c8-8fbe-4397-9dbb-9b2b20218538