Revert of fAddedToCache doesn't need to be atomic. (patchset #1 id:1 of https://coder...

Revert of fAddedToCache doesn't need to be atomic. (patchset #1 id:1 of https://codereview.chromium.org/960573002/)

Reason for revert:
Yes it does.  notifyAddedToCache() must be thread-safe.

Original issue's description:
> fAddedToCache doesn't need to be atomic.
>
> It's only ever read or set from non-threadsafe methods.
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/fbe0edfec4fed2a09e12b049d527d280f16e75b3

TBR=reed@google.com,mtklein@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/959763002