Introduce a FlatMap and FlatSet into WTF

third_party/WebKit/Source/platform/scheduler/base/time_domain.h

Index: third_party/WebKit/Source/platform/scheduler/base/time_domain.h
diff --git a/third_party/WebKit/Source/platform/scheduler/base/time_domain.h b/third_party/WebKit/Source/platform/scheduler/base/time_domain.h
index 52c7678eea23ac5de1257546e25a5172f24ea4fa..d2471e67156801b79cbddc7ba1fc0dedb90cb232 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/time_domain.h
+++ b/third_party/WebKit/Source/platform/scheduler/base/time_domain.h
@@ -15,6 +15,7 @@
 #include "base/time/time.h"
 #include "platform/scheduler/base/lazy_now.h"
 #include "platform/scheduler/base/task_queue_impl.h"
+#include "wtf/FlatSet.h"
 
 namespace blink {
 namespace scheduler {
@@ -134,20 +135,10 @@ class BLINK_PLATFORM_EXPORT TimeDomain {
  private:
   void MoveNewlyUpdatableQueuesIntoUpdatableQueueSet();
 
-  using DelayedWakeupMultimap =
-      std::multimap<base::TimeTicks, internal::TaskQueueImpl*>;
+  using DelayedWakeupSet =
+      WTF::FlatSet<std::pair<base::TimeTicks, internal::TaskQueueImpl*>>;
 
-  DelayedWakeupMultimap delayed_wakeup_multimap_;
-
-  // This map makes it easy to remove a queue from |delayed_wakeup_multimap_|.
-  // NOTE inserting or removing elements from a std::map does not invalidate any
-  // iterators.
-  using QueueToDelayedWakeupMultimapIteratorMap =
-      std::unordered_map<internal::TaskQueueImpl*,
-                         DelayedWakeupMultimap::iterator>;
-
-  QueueToDelayedWakeupMultimapIteratorMap
-      queue_to_delayed_wakeup_multimap_iterator_map_;
+  DelayedWakeupSet delayed_wakeup_set_;
 
   // This lock guards only |newly_updatable_|.  It's not expected to be heavily
   // contended.
@@ -158,6 +149,8 @@ class BLINK_PLATFORM_EXPORT TimeDomain {
   // only be accessed from the main thread.
   std::set<internal::TaskQueueImpl*> updatable_queue_set_;
 
+  std::set<internal::TaskQueueImpl*> queues_;
+
   Observer* observer_;  // NOT OWNED.
 
   base::ThreadChecker main_thread_checker_;