TaskScheduler: Change Sequence::PeekTask to Sequence::TakeTask.

base/task_scheduler/sequence.h

Index: base/task_scheduler/sequence.h
diff --git a/base/task_scheduler/sequence.h b/base/task_scheduler/sequence.h
index 8717336080e623dfde5ce8e5acc108ebcc9e61c0..408d99f9c6426167cd44ffad579c9fa2555e924b 100644
--- a/base/task_scheduler/sequence.h
+++ b/base/task_scheduler/sequence.h
@@ -22,7 +22,10 @@
 namespace base {
 namespace internal {
 
-// A sequence holds tasks that must be executed in posting order.
+// A Sequence holds slots each containing up to a single Task that must be
+// executed in posting order.
+//
+// In comments below, an "empty Sequence" is a Sequence with no slot.
 //
 // Note: there is a known refcounted-ownership cycle in the Scheduler
 // architecture: Sequence -> Task -> TaskRunner -> Sequence -> ...
@@ -41,20 +44,27 @@ class BASE_EXPORT Sequence : public RefCountedThreadSafe<Sequence> {
  public:
   Sequence();
 
-  // Adds |task| at the end of the sequence's queue. Returns true if the
-  // sequence was empty before this operation.
+  // Adds |task| in a new slot at the end of the Sequence. Returns true if the
+  // Sequence was empty before this operation.
   bool PushTask(std::unique_ptr<Task> task);
 
-  // Returns the task in front of the sequence's queue, if any.
-  const Task* PeekTask() const;
+  // Transfers ownership of the Task in the front slot of the Sequence to the
+  // caller. The front slot of the Sequence will be nullptr and remain until
+  // Pop(). Cannot be called on an empty Sequence or a Sequence whose front slot
+  // is already nullptr.
+  std::unique_ptr<Task> TakeTask();
+
+  // Returns the TaskTraits of the Task in front of the Sequence. Cannot be
+  // called on an empty Sequence or on a Sequence whose front slot is empty.
+  TaskTraits PeekTaskTraits() const;
 
-  // Removes the task in front of the sequence's queue. Returns true if the
-  // sequence is empty after this operation. Cannot be called on an empty
-  // sequence.
-  bool PopTask();
+  // Removes the front slot of the Sequence. The front slot must have been
+  // emptied by TakeTask() before this is called. Cannot be called on an empty
+  // Sequence. Returns true if the Sequence is empty after this operation.
+  bool Pop();
 
-  // Returns a SequenceSortKey representing the priority of the sequence. Cannot
-  // be called on an empty sequence.
+  // Returns a SequenceSortKey representing the priority of the Sequence. Cannot
+  // be called on an empty Sequence.
   SequenceSortKey GetSortKey() const;
 
   // Returns a token that uniquely identifies this Sequence.
@@ -72,7 +82,7 @@ class BASE_EXPORT Sequence : public RefCountedThreadSafe<Sequence> {
   // Queue of tasks to execute.
   std::queue<std::unique_ptr<Task>> queue_;
 
-  // Number of tasks contained in the sequence for each priority.
+  // Number of tasks contained in the Sequence for each priority.
   size_t num_tasks_per_priority_[static_cast<int>(TaskPriority::HIGHEST) + 1] =
       {};