Surfaceless contexts are emulated in the frontend so this extension
can be enabled for the metal backend. It's needed to run Chrome
with ANGLE's metal backend.

Bug: angleproject:4847
Change-Id: I3a2f74fd5f2ac75a056c528c8b30ffbd41d73e46
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2332880Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>