Fix classes that have too many virtuals for inline constructors.

components/drive/dummy_file_system.cc

 // Copyright 2014 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.
 
 #include "components/drive/dummy_file_system.h"
 
 namespace drive {
 
+DummyFileSystem::DummyFileSystem() = default;
+
 base::Closure DummyFileSystem::GetFileContent(
       const base::FilePath& file_path,
       const GetFileContentInitializedCallback& initialized_callback,
       const google_apis::GetContentCallback& get_content_callback,
       const FileOperationCallback& completion_callback) {
   return base::Bind(&base::DoNothing);
 }
 
 }  // namespace drive