Make tasks cancellable inside the blink scheduler.

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

Index: third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h
diff --git a/third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h b/third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h
index 6318555718ac07995c56513f46a3c797a4adf019..6b0e76bbdd83929adc6ced93e4ea6afa8eaea9ac 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h
+++ b/third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h
@@ -15,13 +15,21 @@ 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_;