Add an ASSERT that the new repaint rectanges are correct

Note, this ASSERT will fire when repaint-after-layout is 
enabled but it is important to land so we can fix any
incorrect behaviour.

BUG=320139

Review URL: https://codereview.chromium.org/158923003

git-svn-id: svn://svn.chromium.org/blink/trunk@166861 bbb929c8-8fbe-4397-9dbb-9b2b20218538