assertion failures in WebGL2 CTS.

This reverts commit 54480961.

Change-Id: I2e0c7045c5b6ef9031a6e6c5916504fe68f51077
Reviewed-on: https://chromium-review.googlesource.com/287910Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>