It's possible for multiple threads to touch mFenceRecycler so need to
protect it with a mutex to prevent race conditions.

Bug: b/168744476
Change-Id: I9898d2e9ebf0974581a1074cbda5be41a09cd790
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2415401Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>