UMA stats instrumentation for Renderer.AcceleratedFixedRootBackground

Along with http://crrev.com/23983017, this adds and makes use of
Renderer.AcceleratedFixedRootBackground UMA histogram.

Records three counts:
1. The total number of main frame scrolls.
2. The total number of main frame scrolls with an accelerated fixed root background.
3. The total number of main frame scrolls with an unaccelerated fixed root background.

R=hartmanng@chromium.org,asvitkine@chromium.org
BUG=None

Review URL: https://chromiumcodereview.appspot.com/23458035

git-svn-id: svn://svn.chromium.org/blink/trunk@157457 bbb929c8-8fbe-4397-9dbb-9b2b20218538