Make tasks cancellable inside the blink scheduler.

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

Index: third_party/WebKit/Source/platform/scheduler/base/work_queue_sets.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/base/work_queue_sets.cc b/third_party/WebKit/Source/platform/scheduler/base/work_queue_sets.cc
index 47ffb0e8877e176d302872447602ef40de86b246..f11d3457cbcc3c3b2e1a85db5a0273430e1ccfe0 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/work_queue_sets.cc
+++ b/third_party/WebKit/Source/platform/scheduler/base/work_queue_sets.cc
@@ -75,6 +75,7 @@ void WorkQueueSets::OnPushQueue(WorkQueue* work_queue) {
 }
 
 void WorkQueueSets::OnPopQueue(WorkQueue* work_queue) {
+  // Assume that |work_queue| contains the lowest enqueue_order.
   size_t set_index = work_queue->work_queue_set_index();
   DCHECK_EQ(this, work_queue->work_queue_sets());
   DCHECK_LT(set_index, enqueue_order_to_work_queue_maps_.size());
@@ -94,6 +95,26 @@ void WorkQueueSets::OnPopQueue(WorkQueue* work_queue) {
       std::make_pair(enqueue_order, work_queue));
 }
 
+void WorkQueueSets::OnQueueHeadChanged(WorkQueue* work_queue,
+                                       EnqueueOrder erased_task_enqueue_order) {
+  size_t set_index = work_queue->work_queue_set_index();
+  DCHECK_EQ(this, work_queue->work_queue_sets());
+  DCHECK_LT(set_index, enqueue_order_to_work_queue_maps_.size());
+  DCHECK(!enqueue_order_to_work_queue_maps_[set_index].empty())
+      << " set_index = " << set_index;
+  DCHECK(enqueue_order_to_work_queue_maps_[set_index].find(
+             erased_task_enqueue_order) !=
+         enqueue_order_to_work_queue_maps_[set_index].end());
+  // O(log n).
+  enqueue_order_to_work_queue_maps_[set_index].erase(erased_task_enqueue_order);
+  EnqueueOrder enqueue_order;
+  bool has_enqueue_order = work_queue->GetFrontTaskEnqueueOrder(&enqueue_order);
+  if (!has_enqueue_order)
+    return;
+  enqueue_order_to_work_queue_maps_[set_index].insert(
+      std::make_pair(enqueue_order, work_queue));
+}
+
 bool WorkQueueSets::GetOldestQueueInSet(size_t set_index,
                                         WorkQueue** out_work_queue) const {
   DCHECK_LT(set_index, enqueue_order_to_work_queue_maps_.size());