Make defaultBackendFormat callable from anywhere in the GrContext hierarchy

This is probably overkill but yields an officially thread-safe means of calling defaultBackendFormat.

Change-Id: Ibf344acf11102d938e335e9ea38bfaee575e00c5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/234324
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed