Kill the "stacking container" concept.

When we switch to "universal" accelerated overflow
scrolling, this is no longer necessary.

BUG=249354

Review URL: https://codereview.chromium.org/261503002

git-svn-id: svn://svn.chromium.org/blink/trunk@174191 bbb929c8-8fbe-4397-9dbb-9b2b20218538