This doesn't need to be static as the whole SamplerManager class has
only one static instance anyway, via instance().

This might also fix the weird lock behavior we are seeing on Nexus 5x
where the atomic_bool lock seems to never be released.

Bug: v8:8649
Change-Id: If44b6361c9e2a124265ca5b15b997538475a2ec9
Reviewed-on: https://chromium-review.googlesource.com/c/1414854Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58847}