Use emulatedEtc1 limitation to hide it from WebGL.

Bug: chromium:1048244
Change-Id: Id2fd6b5d934f3caeb455e8f0fc198445e8c00084
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2803136Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>