Use "universal" accelerated scrolling exclusively on bleeding edge.

Currently, the universal path depends on the legacy path. The reason was
that the legacy path created fewer layers (by forcing stacking context-
like behavior). In the world of squashing, this is dumb. The legacy path
requires some very expensive opt-in checks that we'd like to avoid. This
patch enables the universal path, and only the universal path, when
we're in bleeding-edge mode, skipping the legacy work.

(Aside: the stacking container concept is not long for this world. \o/).

BUG=None

Review URL: https://codereview.chromium.org/231163002

git-svn-id: svn://svn.chromium.org/blink/trunk@171166 bbb929c8-8fbe-4397-9dbb-9b2b20218538