remote fonts: Make DiscardableManager for tests thread-safe.

R=herb@google.com, mtklein@google.com

Bug: skia:8194
Change-Id: I25084278d41528cfe790bcdb108b28d624a32c73
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201686
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Auto-Submit: Khushal Sagar <khushalsagar@chromium.org>
1 file changed