Add GrContext API to report the number of purgeable bytes in the cache.

Change-Id: I1457eec9831736f386e3b3b80d9eac8dbb337a9b
Reviewed-on: https://skia-review.googlesource.com/17829
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Derek Sollenberger <djsollen@google.com>
5 files changed