Fix restrictions on squashing with incompatible scroll.

The previous logic could fail in some cases when the heuristic specified for the identity of the layer to squash
into did not match exactly the choice used later in assignLayersToBackings. The new code is a bit cleaner also.

BUG=261605

Review URL: https://codereview.chromium.org/195893026

git-svn-id: svn://svn.chromium.org/blink/trunk@169571 bbb929c8-8fbe-4397-9dbb-9b2b20218538