[DevTools] migrate InspectorDOMAgent to new style

ash/common/devtools/ash_devtools_dom_agent.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 ASH_COMMON_DEVTOOLS_ASH_DEVTOOLS_DOM_AGENT_H_
 #define ASH_COMMON_DEVTOOLS_ASH_DEVTOOLS_DOM_AGENT_H_
 
 #include "ash/common/wm_shell.h"
 #include "components/ui_devtools/DOM.h"
 #include "components/ui_devtools/devtools_base_agent.h"
 
 namespace ash {
 namespace devtools {
 
 class AshDevToolsDOMAgent : public ui::devtools::UiDevToolsBaseAgent<
                                 ui::devtools::protocol::DOM::Metainfo> {
  public:
   explicit AshDevToolsDOMAgent(ash::WmShell* shell);
   ~AshDevToolsDOMAgent() override;
 
  private:
   std::unique_ptr<ui::devtools::protocol::DOM::Node> BuildInitialTree();
 
   // DOM::Backend
-  void enable(ui::devtools::protocol::ErrorString* error) override {}
-  void disable(ui::devtools::protocol::ErrorString* error) override {}
-  void getDocument(
-      ui::devtools::protocol::ErrorString* error,
+  ui::devtools::protocol::Response enable() override;
+  ui::devtools::protocol::Response disable() override;
+  ui::devtools::protocol::Response getDocument(
       std::unique_ptr<ui::devtools::protocol::DOM::Node>* out_root) override;
 
   ash::WmShell* shell_;
 
   DISALLOW_COPY_AND_ASSIGN(AshDevToolsDOMAgent);
 };
 
 }  // namespace devtools
 }  // namespace ash
 
 #endif  // ASH_COMMON_DEVTOOLS_ASH_DEVTOOLS_DOM_AGENT_H_