Fully update compositing information for *layerTreeAsText calls

This patch ensures the compositing layers are up-to-date
when calling elementLayerTreeAsText and layerTreeAsText
from tests. Before, elementLayerTreeAsText had a
DisableCompositingQueryAsserts which this patch removes.
layerTreeAsText previously updated updateCompositingLayers
but that was not sufficient; this patch adds a tighter
assert and calls updateLayoutAndStyleForPainting to fully
update.

BUG=

Review URL: https://codereview.chromium.org/202523004

git-svn-id: svn://svn.chromium.org/blink/trunk@169579 bbb929c8-8fbe-4397-9dbb-9b2b20218538