Replace the gc extension in content shell with a gin::Wrappable class

content/shell/renderer/gc_extension.h

-// Copyright 2013 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 CONTENT_SHELL_RENDERER_GC_EXTENSION_H_
-#define CONTENT_SHELL_RENDERER_GC_EXTENSION_H_
-
-namespace v8 {
-class Extension;
-}
-
-namespace extensions_v8 {
-
-// GCExtension is a v8 extension to expose a method into JS for triggering
-// garbage collection. This should only be used for debugging.
-class GCExtension {
- public:
-  static v8::Extension* Get();
-};
-
-}  // namespace extensions_v8
-
-#endif  // CONTENT_SHELL_RENDERER_GC_EXTENSION_H_