Add arc support to gpu Obj c++ code

This is mainly for getting ready to start adding lots of metal backend code.
I've also update the "gpu tools" target to require ARC with involved updating
one IOS file in there.

Bug: skia:
Change-Id: Ied22e8fe7532445cc274efb529e3450654a6614b
Reviewed-on: https://skia-review.googlesource.com/22484
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
5 files changed