Remove isCompositorFramePending plumbing

Originally added: https://codereview.chromium.org/139103009.

This is only used for one test, and is somewhat a lie because
WebTestProxy implements its own (very different) animate/composite
scheduling.  Removing this plumbing makes it possible to better clean up
old non-composited paint logic inside of WebTestProxy.

Due to changes elsewhere, this test no longer catches the original
regression.  The scheduleAnimate call inside of RenderLayerCompositor
that was added in the original patch can be removed and the page still
paints properly.  In fact, all scheduleAnimate calls and calls to the
page animator in RLC can be removed and the page still gets updated.

The code in RenderLayerCompositor has changed drastically since m33, and
if possible I'd like to remove this one-off plumbing that is no longer
catching any regressions.

TBR=abarth@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/289873002

git-svn-id: svn://svn.chromium.org/blink/trunk@174200 bbb929c8-8fbe-4397-9dbb-9b2b20218538