Introduce a FlatMap and FlatSet into WTF

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 194524f1740f557c25154ab3aeed6097d013fef3..94c39add872cf50643d73b9c5a85f3625483fb3a 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
@@ -25,7 +25,7 @@ void WorkQueueSets::AddQueue(WorkQueue* work_queue, size_t set_index) {
   work_queue->AssignSetIndex(set_index);
   if (!has_enqueue_order)
     return;
-  enqueue_order_to_work_queue_maps_[set_index].insert(
+  enqueue_order_to_work_queue_maps_[set_index].insertUnique(
       std::make_pair(enqueue_order, work_queue));
 }
 
@@ -56,7 +56,7 @@ void WorkQueueSets::ChangeSetIndex(WorkQueue* work_queue, size_t set_index) {
   if (!has_enqueue_order)
     return;
   enqueue_order_to_work_queue_maps_[old_set].erase(enqueue_order);
-  enqueue_order_to_work_queue_maps_[set_index].insert(
+  enqueue_order_to_work_queue_maps_[set_index].insertUnique(
       std::make_pair(enqueue_order, work_queue));
 }
 
@@ -70,7 +70,7 @@ void WorkQueueSets::OnPushQueue(WorkQueue* work_queue) {
   size_t set_index = work_queue->work_queue_set_index();
   DCHECK_LT(set_index, enqueue_order_to_work_queue_maps_.size())
       << " set_index = " << set_index;
-  enqueue_order_to_work_queue_maps_[set_index].insert(
+  enqueue_order_to_work_queue_maps_[set_index].insertUnique(
       std::make_pair(enqueue_order, work_queue));
 }
 
@@ -84,17 +84,14 @@ void WorkQueueSets::OnPopQueue(WorkQueue* work_queue) {
   DCHECK_EQ(enqueue_order_to_work_queue_maps_[set_index].begin()->second,
             work_queue)
       << " set_index = " << set_index;
-  EnqueueOrderToWorkQueueMap::iterator old_it =
-      enqueue_order_to_work_queue_maps_[set_index].begin();
   EnqueueOrder enqueue_order;
   if (work_queue->GetFrontTaskEnqueueOrder(&enqueue_order)) {
-    // Amortized O(1) if the new location is close to |old_it|, otherwise
-    // O(log n).
-    enqueue_order_to_work_queue_maps_[set_index].insert(
-        std::make_pair(enqueue_order, work_queue));
+    enqueue_order_to_work_queue_maps_[set_index].ChangeKeyUnique(
+        enqueue_order_to_work_queue_maps_[set_index].begin(), enqueue_order);
+  } else {
+    enqueue_order_to_work_queue_maps_[set_index].erase(
+        enqueue_order_to_work_queue_maps_[set_index].begin());
   }
-  // O(1)
-  enqueue_order_to_work_queue_maps_[set_index].erase(old_it);
 }
 
 bool WorkQueueSets::GetOldestQueueInSet(size_t set_index,