Get rid of RenderLayerCompositor::hasUnresolvedDirtyBits.

Instead, assert that we don't have unresolved dirty bits.
We still need to check m_needsToRecomputeCompositingRequirements,
but there's already FIXMEs to get rid of that. Once we
do, we can get rid of scheduleAnimationIfNeeded entirely,
making it so that we never schedule an extra animation as
a result of the compositing update itself.

BUG=354100

Review URL: https://codereview.chromium.org/295533002

git-svn-id: svn://svn.chromium.org/blink/trunk@174300 bbb929c8-8fbe-4397-9dbb-9b2b20218538