Allow layers that need squashing to draw into the squashing layer in some cases.

In particular, allow a layer L to squash if their clipping
ancestor differs from the squashing owner, but the clipping
ancestor for L is a descendant of a layer that does squash
into the squashing owner.

BUG=368410

Review URL: https://codereview.chromium.org/264013002

git-svn-id: svn://svn.chromium.org/blink/trunk@173690 bbb929c8-8fbe-4397-9dbb-9b2b20218538