Disable layer squashing

We've gotten a number of interesting bug reports. We're going to disable this
feature on trunk while we work through the bug reports.

This CL adds a new way to disable layer squashing via RuntimeEnabledFeatures.
This mechanism lets us leave the feature turned on for the LayoutTests so we
don't need to keep rebaselining them when we turn the feature on and off.

R=chrishtr@chromium.org
BUG=365701

Review URL: https://codereview.chromium.org/246893017

git-svn-id: svn://svn.chromium.org/blink/trunk@172405 bbb929c8-8fbe-4397-9dbb-9b2b20218538