Remove SkImage_Base::asTextureRef (in favor of asTextureProxyRef)

Minor consolidation/clean-up

Change-Id: I9f8ba794cfb95b33dd60ada0e734ddd2f5f21710
Reviewed-on: https://skia-review.googlesource.com/9947
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed