Make tasks cancellable inside the blink scheduler.

third_party/WebKit/Source/platform/scheduler/base/enqueue_order.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 THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_ENQUEUE_ORDER_H_
 #define THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_ENQUEUE_ORDER_H_
 
 #include <stdint.h>
 
 #include "base/synchronization/lock.h"
 
 namespace blink {
 namespace scheduler {
 namespace internal {
 
 using EnqueueOrder = uint64_t;
 
+// A 64bit integer used to provide ordering of tasks. NOTE The scheduler assumes
+// these values will not overflow.
 class EnqueueOrderGenerator {
  public:
   EnqueueOrderGenerator();
   ~EnqueueOrderGenerator();
 
+  // Returns a monotonically increasing integer, starting from one. Can be
+  // called from any thread.
   EnqueueOrder GenerateNext();
 
+  static bool IsValidEnqueueOrder(EnqueueOrder enqueue_order) {
+    return enqueue_order != 0ull;
+  }
+
  private:
   base::Lock lock_;
   EnqueueOrder enqueue_order_;
 };
 
 }  // namespace internal
 }  // namespace scheduler
 }  // namespace blink
 
 #endif  // THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_ENQUEUE_ORDER_H_