Don't squash fixed-position elements into another fixed-position element that has an
overflow clip.

Previously it checked for clip but not overflow clip.

BUG=409680

Review URL: https://codereview.chromium.org/543123003

git-svn-id: svn://svn.chromium.org/blink/trunk@181495 bbb929c8-8fbe-4397-9dbb-9b2b20218538