SkDiscardableMemoryPool: always have a mutex for testing

Change-Id: I898058f1125f62832f2b2b661c661efb0f84da6d
Reviewed-on: https://skia-review.googlesource.com/14274
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Herb Derby <herb@google.com>
6 files changed