Enable layer squashing

This is a second attempt to enable layer squashing. Rather than enabling this
from the Chromium side, this CL enables it on the Blink side. Once this lands,
we can switch over to controlling this from the Chromium side so that
--disable-layer-squashing works.

Review URL: https://codereview.chromium.org/242373004

git-svn-id: svn://svn.chromium.org/blink/trunk@171962 bbb929c8-8fbe-4397-9dbb-9b2b20218538