Make compressedBackendFormat accessible from GrContextThreadSafeProxy

Not having this available on GrContextThreadSafeProxy was an oversight.
This will allow Chrome to correctly prepare the correct type of compressed promise image.

Change-Id: I3002609fef00a130fe409f318ae0f442ef7591d6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/437016
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed