WebGPU: blocklist the OpenGLES backend on Android.

This is to prevent a compromised renderer from gaining access to
the OpenGLES backend until further testing and validation has been
performed.

(Note: this is pre-emptive, since the OpenGLES backend has not yet
been enabled upstream in Dawn.)

Bug: 40162674
Change-Id: I8d38e858dfc5efa3ed75b5701208b72467215af9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5426402


Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283509}