Surfaces should acknowledge frame submissions

Previously, there was no way for clients of the surfaces service to
appropriately rate-limit their frame submissions. We tried using the "return
resources" signal to rate-limit, but that's really a measure of how quickly
you're submitting new resources rather than how quickly the system is putting
up frames.

Currently, only the Sky compositor listens to this signal. Using this signal,
we're able to run sky/examples/spinning-square.sky in sync with the surfaces
service (that is, submitting exactly one frames every 17ms).

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/756673004