[test] Fix timeout in AddTraceEventMultiThreaded

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}

test/cctest/libplatform/test-tracing.cc

@@ -477,8 +477,7 @@ class TraceWritingThread : public base::Thread {
         tracing_controller_(tracing_controller) {}
 
   void Run() override {
-    running_.store(true);
-    while (running_.load()) {
+    while (!stopped_.load()) {
       TRACE_EVENT0("v8", "v8.Test");
       tracing_controller_->AddTraceEvent('A', nullptr, "v8", "", 1, 1, 0,
                                          nullptr, nullptr, nullptr, nullptr, 0);
@@ -488,10 +487,10 @@ class TraceWritingThread : public base::Thread {
     }
   }
 
-  void Stop() { running_.store(false); }
+  void Stop() { stopped_.store(true); }
 
  private:
-  std::atomic_bool running_{false};
+  std::atomic_bool stopped_{false};
   v8::platform::tracing::TracingController* tracing_controller_;
 };