Make tasks cancellable inside the blink scheduler.

third_party/WebKit/Source/platform/scheduler/base/enqueue_order.cc

 // 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.
 
 #include "platform/scheduler/base/enqueue_order.h"
 
 namespace blink {
 namespace scheduler {
 namespace internal {
 
-EnqueueOrderGenerator::EnqueueOrderGenerator() : enqueue_order_(0) {}
+// Note we set the first |enqueue_order_| to 1 so that isValid may distinguish a
+// valid EnqueueOrder from an invalid one.
+EnqueueOrderGenerator::EnqueueOrderGenerator() : enqueue_order_(1) {}
 
 EnqueueOrderGenerator::~EnqueueOrderGenerator() {}
 
 EnqueueOrder EnqueueOrderGenerator::GenerateNext() {
   base::AutoLock lock(lock_);
   return enqueue_order_++;
 }
 
 }  // namespace internal
 }  // namespace scheduler
 }  // namespace blink