scheduler: Maintain a constant enqueue order for every task

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 04932d54376478f53556d100ef23692280bee6ff..e4073e6297b1b1009b9e51b810392b476637486e 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h
+++ b/third_party/WebKit/Source/platform/scheduler/base/enqueue_order.h
@@ -8,41 +8,54 @@
 #include <stdint.h>
 
 #include "base/synchronization/lock.h"
+#include "public/platform/WebCommon.h"
 
 namespace blink {
 namespace scheduler {
 namespace internal {
 
-using EnqueueOrder = uint64_t;
+using SequenceNumber = uint64_t;
 
-// TODO(scheduler-dev): Remove explicit casts when c++17 comes.
-enum class EnqueueOrderValues : EnqueueOrder {
-  // Invalid EnqueueOrder.
+// The enqueue order is used to uniquely order tasks. The primary ordering key
+// is the desired run time of a task, followed by the sequence number. Note the
+// scheduler assumes sequence numbers will not overflow.
+struct BLINK_PLATFORM_EXPORT EnqueueOrder {
+  base::TimeTicks delayed_run_time;
+  SequenceNumber sequence_num;
+
+  EnqueueOrder();
+  EnqueueOrder(base::TimeTicks delayed_run_time, SequenceNumber sequence_num);
+
+  bool is_null() const { return delayed_run_time.is_null() && !sequence_num; }
+
+  bool operator==(const EnqueueOrder& other) const;
+  bool operator<=(const EnqueueOrder& other) const;
+  bool operator<(const EnqueueOrder& other) const;
+};
+
+std::ostream& operator<<(std::ostream&, const EnqueueOrder&);
+
+enum class EnqueueOrderSequenceNumberValues {
+  // Invalid sequence number.
   NONE = 0,
 
-  // Earliest possible EnqueueOrder, to be used for fence blocking.
+  // Earliest possible sequence number, to be used for fence blocking.
   BLOCKING_FENCE = 1,
   FIRST = 2,
 };
 
-// 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
+  // Returns a monotonically increasing enqueue order, starting from one. Can be
   // called from any thread.
-  EnqueueOrder GenerateNext();
-
-  static bool IsValidEnqueueOrder(EnqueueOrder enqueue_order) {
-    return enqueue_order != 0ull;
-  }
+  EnqueueOrder GenerateNext(base::TimeTicks delayed_run_time);
 
  private:
   base::Lock lock_;
-  EnqueueOrder enqueue_order_;
+  SequenceNumber sequence_num_;
 };
 
 }  // namespace internal