Add backend GPU objects to fiddle app

TBR=bsalomon@google.com
Change-Id: I8876a4657f837436322150925233e0f36c91e8f0
Reviewed-on: https://skia-review.googlesource.com/72641
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
8 files changed