Only skip repaints for layers whose bookkeeping is up to date

We currently skip full repaints for scrollable areas if everything that
moves in response to a scroll offset update is a composited layer.
We only do the bookkeeping to make that determination for scrollable
areas that are registered in the FrameView's set of scrollable areas.
It turns out that overflow:hidden layers are not included in this set,
so their answer to the question 'is-everything-that-scrolls-composited'
will be wrong. This CL ensures that we only skip this work for layers
we've done bookkeeping for.

R=enne@chromium.org

TEST=compositing/overflow/do-not-repaint-if-scrolling-composited-layers.html
BUG=319655

Review URL: https://codereview.chromium.org/79263002

git-svn-id: svn://svn.chromium.org/blink/trunk@162402 bbb929c8-8fbe-4397-9dbb-9b2b20218538