MockTaskRunner is missing a lock to protect tasks queue, causing flaky
tsan. This is similar to:
https://source.chromium.org/chromium/chromium/src/+/master:v8/test/cctest/wasm/test-wasm-metrics.cc;l=94?q=test%2Fcctest%2Fwasm%2Ftest-wasm-metrics.cc&ss=chromium

Previous CL https://chromium-review.googlesource.com/c/v8/v8/+/2507379
probably revealed the issue to TSAN by bringing more tasks.

Bug: v8:11194
Change-Id: Ib45c4afb4e7a86c9b4a54518876e311598747919
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2555383Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71346}