[d8] Fix data race in DelayedTasksPlatform

Reported here:
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20NumFuzz%20-%20TSAN/3108

R=yangguo@chromium.org

Bug: v8:8278
Change-Id: I78d13cea592678c1ab73b61fd9738f375df565d0
Reviewed-on: https://chromium-review.googlesource.com/c/1276631Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56576}

src/d8-platforms.cc

@@ -230,6 +230,7 @@ class DelayedTasksPlatform : public Platform {
    public:
     void operator()(DelayedTaskRunner* runner) const {
       TaskRunner* original_runner = runner->task_runner_.get();
+      base::LockGuard<base::Mutex> lock_guard(&runner->platform_->mutex_);
       auto& delayed_task_runners = runner->platform_->delayed_task_runners_;
       DCHECK_EQ(1, delayed_task_runners.count(original_runner));
       delayed_task_runners.erase(original_runner);