Enable universal accelerated overflow scroll

Now that we've enabled layer squashing, we can enable universal overflow
scrolling.

Note: This is just an updated version of the patch abarth@ kindly
started here: https://codereview.chromium.org/243713002/

I've removed some now meaningless layout tests (which cover the old,
opt-in path), and updated the TestExpectaions for the remaining,
affected tests.

R=abarth@chromium.org
BUG=249354

Review URL: https://codereview.chromium.org/244193002

git-svn-id: svn://svn.chromium.org/blink/trunk@172041 bbb929c8-8fbe-4397-9dbb-9b2b20218538