WebGL 2.0 spec section 5.25 specifies this.

BUG=angleproject:1542
TEST=WebGL 2 conformance tests

Change-Id: Ic5758bc0e767a8e6dd032e42c0a69bcae06381c4
Reviewed-on: https://chromium-review.googlesource.com/399682Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>