[Extensions Bindings] Add a registerEventArgumentMassager

extensions/renderer/api_event_handler.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_API_EVENT_HANDLER_H_
 #define EXTENSIONS_RENDERER_API_EVENT_HANDLER_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 26 matching lines @@
   // Returns a new v8::Object for an event with the given |event_name|.
   v8::Local<v8::Object> CreateEventInstance(const std::string& event_name,
                                             v8::Local<v8::Context> context);
 
   // Notifies all listeners of the event with the given |event_name| in the
   // specified |context|, sending the included |arguments|.
   void FireEventInContext(const std::string& event_name,
                           v8::Local<v8::Context> context,
                           const base::ListValue& arguments);
 
+  // Registers a |function| to serve as an "argument massager" for the given
+  // |event_name|, mutating the original arguments.
+  // The function is called with two arguments: the array of original arguments
+  // being dispatched to the event, and the function to dispatch the event to
+  // listeners.
+  void RegisterArgumentMassager(v8::Local<v8::Context> context,
+                                const std::string& event_name,
+                                v8::Local<v8::Function> function);
+
   // Returns the EventListeners for a given |event_name| and |context|.
   size_t GetNumEventListenersForTesting(const std::string& event_name,
                                         v8::Local<v8::Context> context);
 
  private:
   // Method to run a given v8::Function. Curried in for testing.
   binding::RunJSFunction call_js_;
 
   EventListenersChangedMethod listeners_changed_;
 
   DISALLOW_COPY_AND_ASSIGN(APIEventHandler);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_API_EVENT_HANDLER_H_