GitLab

When a parent compositor is present (ie, ubercompositor), throttle to a single
pending frame.

Tests:
LayerTreeHostTestMaxPendingFrames.DelegatingRenderer
LayerTreeHostTestMaxPendingFrames.GLRenderer

BUG=123444
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/11747002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175275 0039d316-1c4b-4281-b951-d872f2087c98