Adding pinning to the strike cache

Here is a simple API for the object that pins, and
how it flows through the cache.

BUG=skia:7515

Change-Id: I1b8304dc6f9d8652282300fc1e52d52debb5b6f4
Reviewed-on: https://skia-review.googlesource.com/122500
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed