Enable ASSERT in positionFromPaintInvalidationContainer

We are doing paint invalidations after compositing updates
so nothing blocks enabling it anymore.

BUG=360286

Review URL: https://codereview.chromium.org/479563006

git-svn-id: svn://svn.chromium.org/blink/trunk@180474 bbb929c8-8fbe-4397-9dbb-9b2b20218538