telemetry: Use browser compositor rendering stats in smoothness

Depends on https://codereview.chromium.org/148983007/

Switch smoothness benchmark to use rendering stats from the top level
compositor; i.e. if data from a browser compositor is available, use that.
Otherwise use the data from the renderer compositor.

R=nduca@chromium.org,tonyg@chromium.org
BUG=340753

Review URL: https://codereview.chromium.org/150103003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249523 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249523