[Extensions Binding] Allow for registering custom hooks

extensions/renderer/api_binding_types.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_BINDING_TYPES_H_
 #define EXTENSIONS_RENDERER_API_BINDING_TYPES_H_
 
+#include <memory>
+#include <vector>
+
 #include "base/callback.h"
 #include "v8/include/v8.h"
 
 namespace extensions {
+class ArgumentSpec;
+
 namespace binding {
 
 // A callback to execute the given v8::Function with the provided context and
 // arguments.
 using RunJSFunction = base::Callback<void(v8::Local<v8::Function>,
                                           v8::Local<v8::Context>,
                                           int argc,
                                           v8::Local<v8::Value>[])>;
 
+using APISignature = std::vector<std::unique_ptr<ArgumentSpec>>;
+
 }  // namespace binding
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_API_BINDING_TYPES_H_