Disables lots of assertion failures in WebGL CTS.

BUG=angleproject:882

Change-Id: I407397a256955d608dbf1234e4e391704653bb5a
Reviewed-on: https://chromium-review.googlesource.com/273547Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>