[M149] Guard RestoreTokenManager add/reads with Mutex

Original change's description:
> Guard RestoreTokenManager add/reads with Mutex
>
> The RestoreTokenManager is a singleton who's adds and reads could be
> accessed by multiple threads simultaneously. To prevent any potential
> issues/collisions, ensure that such accesses happen behind a lock.
>
> Bug: chromium:513049286
> Change-Id: I44c1c7977a6d02e1ac3fbe00e6ffc7bc22e41e46
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/472480
> Reviewed-by: Mark Foltz <mfoltz@chromium.org>
> Auto-Submit: Alexander Cooper <alcooper@chromium.org>
> Commit-Queue: Mark Foltz <mfoltz@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#47711}

(cherry picked from commit b9f0b184cb5a487fcb4910efaf4f84483ddba284)

Bug: chromium:514929759,chromium:513049286
Change-Id: I44c1c7977a6d02e1ac3fbe00e6ffc7bc22e41e46
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474080
Commit-Queue: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Chrome Cherry Picker <chrome-cherry-picker@chops-service-accounts.iam.gserviceaccount.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/7827@{#2}
Cr-Branched-From: d606bc991592fbf4dcbe85e9d05db5e501a5ad42-refs/heads/main@{#47595}