Compute squashing offsets from the transform ancestor, not absolute.

This is more efficient, though the previous code does seem to have been correct.

BUG=366080

Review URL: https://codereview.chromium.org/254883002

git-svn-id: svn://svn.chromium.org/blink/trunk@172685 bbb929c8-8fbe-4397-9dbb-9b2b20218538