[Extensions Bindings] Add a registerEventArgumentMassager

extensions/renderer/api_event_handler.h

Index: extensions/renderer/api_event_handler.h
diff --git a/extensions/renderer/api_event_handler.h b/extensions/renderer/api_event_handler.h
index fbf9b42a7ea742f1accc48e10775af037a6b8e0b..36046c8c23e6a831212f8f0d3b02d6b55ee8b234 100644
--- a/extensions/renderer/api_event_handler.h
+++ b/extensions/renderer/api_event_handler.h
@@ -44,6 +44,15 @@ class APIEventHandler {
                           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);