Make CreateBackendFormatFromTexture shared code

TBR=bsalomon@google.com
Change-Id: I19e6e6c8dc32ba584738545d443de092e4987a5a
Reviewed-on: https://skia-review.googlesource.com/114374
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed