In https://crrev.com/c/v8/v8/+/2536465 we added acquiring the lock in
WriteToFlat. Then, acquiring in TryStringToDouble not only is not
necessary but also has undefined behaviour.

This was causing timeouts and meant the tests were disabled in
https://crrev.com/c/v8/v8/+/2543398.

Bug: v8:7790, v8:11171
Change-Id: Iaab4e5079bac96786e536a2e4b766e93ea17e2c4
Fixes: v8:11171
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2544544Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71242}