Enable paint invalidation container ASSERT

This ASSERT was disabled with repaint-after-layout. Now that we have
moved invalidation after compositing update, we can enable it. The
ASSERT uncovered a bug in
RenderLayer::enclosingLayerForPaintInvalidation where we were walking
over stacking contexts and not just renderers.

BUG=360286

Review URL: https://codereview.chromium.org/414503002

git-svn-id: svn://svn.chromium.org/blink/trunk@178935 bbb929c8-8fbe-4397-9dbb-9b2b20218538