Fix data race for SetAddQueueTimeToTasks().

Switch to an atomic variable to ensure that getter and setter do not have a data
race.

Bug: 859155
Change-Id: Ib5ab1968cc8e25ab363fb4ed0b3f2e756d9cc984
Reviewed-on: https://chromium-review.googlesource.com/1240634
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594079}

base/message_loop/message_loop_task_runner.cc

@@ -108,8 +108,7 @@ void MessageLoopTaskRunner::InjectTask(OnceClosure task) {
 }
 
 void MessageLoopTaskRunner::SetAddQueueTimeToTasks(bool enable) {
-  DCHECK_NE(add_queue_time_to_tasks_, enable);
-  add_queue_time_to_tasks_ = enable;
+  base::subtle::NoBarrier_Store(&add_queue_time_to_tasks_, enable ? 1 : 0);
 }
 
 MessageLoopTaskRunner::~MessageLoopTaskRunner() = default;
@@ -127,7 +126,8 @@ bool MessageLoopTaskRunner::AddToIncomingQueue(const Location& from_here,
 
   PendingTask pending_task(from_here, std::move(task),
                            CalculateDelayedRuntime(from_here, delay), nestable);
-  if (add_queue_time_to_tasks_) {
+
+  if (base::subtle::NoBarrier_Load(&add_queue_time_to_tasks_)) {
     if (pending_task.delayed_run_time.is_null()) {
       pending_task.queue_time = base::TimeTicks::Now();
     } else {

base/message_loop/message_loop_task_runner.h

@@ -5,6 +5,7 @@
 #ifndef BASE_MESSAGE_LOOP_MESSAGE_LOOP_TASK_RUNNER_H_
 #define BASE_MESSAGE_LOOP_MESSAGE_LOOP_TASK_RUNNER_H_
 
+#include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/macros.h"
@@ -105,7 +106,7 @@ class BASE_EXPORT MessageLoopTaskRunner : public SingleThreadTaskRunner,
   int next_sequence_num_ = 0;
 
   // Whether to add the queue time to tasks.
-  bool add_queue_time_to_tasks_ = false;
+  base::subtle::Atomic32 add_queue_time_to_tasks_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(MessageLoopTaskRunner);
 };