Add valid checks in places we query isTestingOnlyBackendTexture

Bug: skia:7477
Change-Id: I410427f12c7bb85d11a5e4ed1f09bbd80bbbb54c
Reviewed-on: https://skia-review.googlesource.com/93000
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed