Add support for tagging GrUniqueKeys with a debug string

Change-Id: Ie7d56214fdee7a19a1e8ca3869e5e4d5e72cedf8
Reviewed-on: https://skia-review.googlesource.com/6632
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed