Allow lazy removal of (context) lifecycle observers while stopping.

third_party/WebKit/Source/platform/LifecycleNotifier.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * Copyright (C) 2013 Google Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 49 matching lines @@
         : m_iterationState(NotIterating)
     {
     }
 
 #if DCHECK_IS_ON()
     T* context() { return static_cast<T*>(this); }
 #endif
 
     using ObserverSet = HeapHashSet<WeakMember<Observer>>;
 
+    void removePending(ObserverSet&);
+
     enum IterationState {
-        AllowingNone = 0,
-        AllowingAddition,
-        AllowingRemoval,
-        NotIterating = AllowingAddition | AllowingRemoval,
+        AllowingNone        = 0,
+        AllowingAddition    = 1,
+        AllowingRemoval     = 2,
+        NotIterating        = AllowingAddition | AllowingRemoval,
+        AllowPendingRemoval = 4,
     };
 
     // Iteration state is recorded while iterating the observer set,
     // optionally barring add or remove mutations.
     IterationState m_iterationState;
     ObserverSet m_observers;
 };
 
 template<typename T, typename Observer>
 inline LifecycleNotifier<T, Observer>::~LifecycleNotifier()
@@ skipping 18 matching lines @@
 template<typename T, typename Observer>
 inline void LifecycleNotifier<T, Observer>::addObserver(Observer* observer)
 {
     RELEASE_ASSERT(m_iterationState & AllowingAddition);
     m_observers.add(observer);
 }
 
 template<typename T, typename Observer>
 inline void LifecycleNotifier<T, Observer>::removeObserver(Observer* observer)
 {
+    // If immediate removal isn't currently allowed,
+    // |observer| is recorded for pending removal.
+    if (m_iterationState & AllowPendingRemoval) {
+        m_observers.add(observer);
+        return;
+    }
     RELEASE_ASSERT(m_iterationState & AllowingRemoval);
     m_observers.remove(observer);
 }
 
+template<typename T, typename Observer>
+inline void LifecycleNotifier<T, Observer>::removePending(ObserverSet& observers)
+{
+    if (m_observers.size()) {
+        // Prevent allocation (==shrinking) while removing;
+        // the table is likely to become garbage soon.
+        ThreadState::NoAllocationScope scope(ThreadState::current());
+        observers.removeAll(m_observers);
+    }
+    m_observers.swap(observers);
+}
+
 } // namespace blink
 
 #endif // LifecycleNotifier_h