Make the no-op happen in the Context, so we can properly generator
more errors for negative WebGL tests. Some validation tests still
need to check for no-ops, such as range checks.

BUG=angleproject:2050

Change-Id: I48d0b3cf640f7f128679600e5df108146cfd6348
Reviewed-on: https://chromium-review.googlesource.com/522869
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>