Get rid of fallback to redraw entire squashing layer on scroll.

This is only needed if universal overflow scrolling is off (and even then,
I can't remember exactly why). Let's get rid of it and not launch squashing
until universal overflow scrolling is done. This code keeps confusing people
about the performance of squashing.

BUG=333012

Review URL: https://codereview.chromium.org/179333006

git-svn-id: svn://svn.chromium.org/blink/trunk@167867 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Layout roots are not necessarily cluster roots.

BUG=332253

Review URL: https://codereview.chromium.org/177533007

git-svn-id: svn://svn.chromium.org/blink/trunk@167868 bbb929c8-8fbe-4397-9dbb-9b2b20218538