message: Add TraceMe::Stop
  Allows stopping the TraceMe before it goes out of scope.
diff: @@ -133,7 +133,10 @@ class TraceMe {
  }
  }
  - ~TraceMe() {
  + // Stop tracing the activity. Called by the destructor, but exposed to allow
  + // stopping tracing before the object goes out of scope. Only has an effect
  + // the first time it is called.
  + void Stop() {
  // We do not need to check the trace level again here.
  // - If tracing wasn't active to start with, we have kUntracedActivity.
  // - If tracing was active and was stopped, we have
  @@ -148,9 +151,12 @@ class TraceMe {
  start_time_, Env::Default()->NowNanos()});
  }
  no_init_.name.~string();
  + start_time_ = kUntracedActivity;
  }
  }
  + ~TraceMe() { Stop(); }
  +
  // TraceMe is not movable or copyable.
  TraceMe(const TraceMe &) = delete;
  TraceMe &operator=(const TraceMe &) = delete;
  