Add accelerated overflow scroll to bleeding edge fast path

Also converts the accelerated scroll related settings
to be compositing triggers as they should have been
originally.

I've removed some histograms as well. They were meant
to track the percentage of opt-in to composited scrolling,
but since we're no longer investing any effort into the
old, opt-in path, we know there will be no progress, so
these statistics don't seem useful.

BUG=None

Review URL: https://codereview.chromium.org/221283008

git-svn-id: svn://svn.chromium.org/blink/trunk@171087 bbb929c8-8fbe-4397-9dbb-9b2b20218538