scheduler: Maintain a constant enqueue order for every task

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

Index: third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc b/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
index f5b49d20381796bb1679ec5adf085c9a48de9460..cbef02f3c299b46c3e2b217d937713948fecdc09 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
+++ b/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
@@ -50,7 +50,8 @@ bool WorkQueue::BlockedByFence() const {
   // If the queue is empty then any future tasks will have a higher enqueue
   // order and will be blocked. The queue is also blocked if the head is past
   // the fence.
-  return work_queue_.empty() || work_queue_.front().enqueue_order() > fence_;
+  return work_queue_.empty() ||
+         work_queue_.front().enqueue_order().sequence_num > fence_;
 }
 
 bool WorkQueue::GetFrontTaskEnqueueOrder(EnqueueOrder* enqueue_order) const {
@@ -67,9 +68,6 @@ bool WorkQueue::GetFrontTaskEnqueueOrder(EnqueueOrder* enqueue_order) const {
 
 void WorkQueue::Push(TaskQueueImpl::Task task) {
   bool was_empty = work_queue_.empty();
-#ifndef NDEBUG
-  DCHECK(task.enqueue_order_set());
-#endif
 
   // Amoritized O(1).
   work_queue_.push_back(std::move(task));
@@ -131,9 +129,11 @@ void WorkQueue::AssignSetIndex(size_t work_queue_set_index) {
   work_queue_set_index_ = work_queue_set_index;
 }
 
-bool WorkQueue::InsertFence(EnqueueOrder fence) {
+bool WorkQueue::InsertFence(SequenceNumber fence) {
   DCHECK_NE(fence, 0u);
-  DCHECK(fence >= fence_ || fence == 1u);
+  DCHECK(fence >= fence_ ||
+         fence == static_cast<SequenceNumber>(
+                      EnqueueOrderSequenceNumberValues::BLOCKING_FENCE));
   bool was_blocked_by_fence = BlockedByFence();
   fence_ = fence;
   // Moving the fence forward may unblock some tasks.
@@ -161,8 +161,8 @@ bool WorkQueue::RemoveFence() {
 bool WorkQueue::ShouldRunBefore(const WorkQueue* other_queue) const {
   DCHECK(!work_queue_.empty());
   DCHECK(!other_queue->work_queue_.empty());
-  EnqueueOrder enqueue_order = 0;
-  EnqueueOrder other_enqueue_order = 0;
+  EnqueueOrder enqueue_order;
+  EnqueueOrder other_enqueue_order;
   bool have_task = GetFrontTaskEnqueueOrder(&enqueue_order);
   bool have_other_task =
       other_queue->GetFrontTaskEnqueueOrder(&other_enqueue_order);