[Extensions Bindings] Notify of event unregistration on invalidation

extensions/renderer/event_emitter.h

 // Copyright 2016 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 EXTENSIONS_RENDERER_EVENT_EMITTER_H_
 #define EXTENSIONS_RENDERER_EVENT_EMITTER_H_
 
 #include <vector>
 
 #include "extensions/renderer/api_binding_types.h"
@@ skipping 22 matching lines @@
 
   static gin::WrapperInfo kWrapperInfo;
 
   // gin::Wrappable:
   gin::ObjectTemplateBuilder GetObjectTemplateBuilder(
       v8::Isolate* isolate) final;
 
   void Fire(v8::Local<v8::Context> context,
             std::vector<v8::Local<v8::Value>>* args);
 
-  Listeners* listeners() { return &listeners_; }
+  // Removes all listeners and marks this object as invalid so that no more
+  // are added.
+  void Invalidate();
+
+  const Listeners* listeners() const { return &listeners_; }
 
  private:
   // Bound methods for the Event JS object.
   void AddListener(gin::Arguments* arguments);
   void RemoveListener(gin::Arguments* arguments);
   bool HasListener(v8::Local<v8::Function> function);
   bool HasListeners();
   void Dispatch(gin::Arguments* arguments);
 
+  // Whether or not this object is still valid; false upon context release.
+  // When invalid, no listeners can be added or removed.
+  bool valid_ = true;
+
   Listeners listeners_;
 
   binding::RunJSFunction run_js_;
 
   ListenersChangedMethod listeners_changed_;
 
   DISALLOW_COPY_AND_ASSIGN(EventEmitter);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_EVENT_EMITTER_H_