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..ab0ea1d5d835d7f1f9d8d7724bddde7c4af98693 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 +15,20 @@ namespace internal {
 
 using EnqueueOrder = uint64_t;
 
+// A 64bit integer used to provide ordering of tasks.

[Sami, 2016/08/18 15:53:43]

Should we also mention we don't support these wrapping around? I'm not sure if
that was the case previously either...

[alex clarke (OOO till 29th), 2016/08/18 16:51:41]

Done.

 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) {

[Sami, 2016/08/18 15:53:43]

nit: IsValidEnqueueOrder

[alex clarke (OOO till 29th), 2016/08/18 16:51:41]

Done.

+    return enqueue_order != 0ull;
+  }
+
  private:
   base::Lock lock_;
   EnqueueOrder enqueue_order_;