Use the correct method for computing a repaint rect that takes into account squashing.

This fixes a mistake in crrev.com/301843002

Includes a test.

BUG=370664

Review URL: https://codereview.chromium.org/306093002

git-svn-id: svn://svn.chromium.org/blink/trunk@175189 bbb929c8-8fbe-4397-9dbb-9b2b20218538