Reland: Explicitly track the scheduler task enqueueing order in a new field

components/scheduler/child/task_queue_sets.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SCHEDULER_CHILD_TASK_QUEUE_SETS_H_
 #define COMPONENTS_SCHEDULER_CHILD_TASK_QUEUE_SETS_H_
 
 #include <map>
 #include <vector>
 
@@ skipping 21 matching lines @@
   void OnPushQueue(internal::TaskQueueImpl* queue);
 
   // If empty it's O(1) amortized, otherwise it's O(log num queues)
   void OnPopQueue(internal::TaskQueueImpl* queue);
 
   // O(1)
   bool GetOldestQueueInSet(size_t set,
                            internal::TaskQueueImpl** out_queue) const;
 
  private:
-  struct AgeComparitor {
-    // The age numbers are generated in sequence.  These will eventually
-    // overflow and roll-over to negative numbers.  We must take care to
-    // preserve the ordering of the map when this happens.
+  struct EnqueueOrderComparitor {
+    // The enqueueorder numbers are generated in sequence.  These will
+    // eventually overflow and roll-over to negative numbers.  We must take care
+    // to preserve the ordering of the map when this happens.
     // NOTE we assume that tasks don't get starved for extended periods so that
     // the task queue ages in a set have at most one roll-over.
     // NOTE signed integer overflow behavior is undefined in C++ so we can't
     // use the (a - b) < 0 trick here, because the optimizer won't necessarily
     // do what we expect.
     // TODO(alexclarke): Consider making age and sequence_num unsigned, because
     // unsigned integer overflow behavior is defined.
     bool operator()(int a, int b) const {
       if (a < 0 && b >= 0)
         return false;
       if (b < 0 && a >= 0)
         return true;
       return a < b;
     }
   };
 
-  typedef std::map<int, internal::TaskQueueImpl*, AgeComparitor> AgeToQueueMap;
-  std::vector<AgeToQueueMap> age_to_queue_maps_;
+  typedef std::map<int, internal::TaskQueueImpl*, EnqueueOrderComparitor>
+      EnqueueOrderToQueueMap;
+  std::vector<EnqueueOrderToQueueMap> enqueue_order_to_queue_maps_;
 
   DISALLOW_COPY_AND_ASSIGN(TaskQueueSets);
 };
 
 }  // namespace internal
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_CHILD_TASK_QUEUE_SETS_H_