Synchronize memory in TaskTracker::Flush().

To prevent races, make sure that code that runs after
TaskTracker::Flush() sees memory changed by all tasks it was waiting
for.

BUG=717527

Review-Url: https://codereview.chromium.org/2857613002
Cr-Commit-Position: refs/heads/master@{#469008}

base/task_scheduler/task_tracker.cc

@@ -215,7 +215,7 @@ void TaskTracker::Shutdown() {
 
 void TaskTracker::Flush() {
   AutoSchedulerLock auto_lock(flush_lock_);
-  while (subtle::NoBarrier_Load(&num_pending_undelayed_tasks_) != 0 &&
+  while (subtle::Acquire_Load(&num_pending_undelayed_tasks_) != 0 &&
          !IsShutdownComplete()) {
     flush_cv_->Wait();
   }
@@ -484,7 +484,7 @@ void TaskTracker::OnBlockingShutdownTasksComplete() {
 
 void TaskTracker::DecrementNumPendingUndelayedTasks() {
   const auto new_num_pending_undelayed_tasks =
-      subtle::NoBarrier_AtomicIncrement(&num_pending_undelayed_tasks_, -1);
+      subtle::Barrier_AtomicIncrement(&num_pending_undelayed_tasks_, -1);
   DCHECK_GE(new_num_pending_undelayed_tasks, 0);
   if (new_num_pending_undelayed_tasks == 0) {
     AutoSchedulerLock auto_lock(flush_lock_);

base/task_scheduler/task_tracker.h

@@ -117,9 +117,9 @@ class BASE_EXPORT TaskTracker {
   const std::unique_ptr<State> state_;
 
   // Number of undelayed tasks that haven't completed their execution. Is
-  // incremented and decremented without a barrier. When it reaches zero,
-  // |flush_lock_| is acquired (forcing memory synchronization) and |flush_cv_|
-  // is signaled.
+  // decremented with a memory barrier after a task runs. Is accessed with an
+  // acquire memory barrier in Flush(). The memory barriers ensure that the
+  // memory written by flushed tasks is visible when Flush() returns.
   subtle::Atomic32 num_pending_undelayed_tasks_ = 0;
 
   // Lock associated with |flush_cv_|. Partially synchronizes access to