Improve GetWeakPtr() to better protect against unsafe usage patterns.

base/observer_list.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_OBSERVER_LIST_H_
 #define BASE_OBSERVER_LIST_H_
 
 #include <stddef.h>
 
 #include <algorithm>
@@ skipping 184 matching lines @@
   EnsureValidIndex();
   DCHECK(list_);
   ++list_->notify_depth_;
 }
 
 template <class ObserverType>
 template <class ContainerType>
 ObserverListBase<ObserverType>::Iter<ContainerType>::~Iter() {
   if (list_ && --list_->notify_depth_ == 0)
     list_->Compact();
+#if DCHECK_IS_ON()
+  if (list_) {
+    auto* list_raw = list_.get();
+    list_ = nullptr;
+    if (!list_raw->HasWeakPtrs()) {
+      list_raw->DetachFromSequence();
+    }
+  }
+#endif
 }
 
 template <class ObserverType>
 template <class ContainerType>
 bool ObserverListBase<ObserverType>::Iter<ContainerType>::operator==(
     const Iter& other) const {
   if (is_end() && other.is_end())
     return true;
   return list_.get() == other.list_.get() && index_ == other.index_;
 }
@@ skipping 125 matching lines @@
   }
 
   bool might_have_observers() const {
     return ObserverListBase<ObserverType>::size() != 0;
   }
 };
 
 }  // namespace base
 
 #endif  // BASE_OBSERVER_LIST_H_