Get rid of RenderLayerCompositor::scheduleAnimationIfNeeded.

This was needed because recomputing the fixed position compositing
reasons could set m_needsToRecomputeCompositingRequirements
during the compositing update itself. So we'd finish the compositing
update and need to schedule another frame to clean it up.

It turns out this was only masking bugs in
CompositingReasonFinder::requiresCompositingForPositionFixed that
are fixed in https://codereview.chromium.org/299343002/.

There's no reason we need to set
m_needsToRecomputeCompositingRequirements if we're already
in a compositing update.

Added some asserts to make sure that we're only ever
hitting this codepath during a compsiting update.

BUG=354100

Review URL: https://codereview.chromium.org/295323003

git-svn-id: svn://svn.chromium.org/blink/trunk@174752 bbb929c8-8fbe-4397-9dbb-9b2b20218538