Initializing running_ to true in Run() could happen after Stop().

Bug: v8:10315
Change-Id: Ibde2ff8cb8a5db862f970df261481cb55f8b6c96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2436459Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70191}