Add test for ash devtools DOM agent getDocument method

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 "base/compiler_specific.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> {
+class ASH_EXPORT AshDevToolsDOMAgent
+    : public NON_EXPORTED_BASE(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,
       std::unique_ptr<ui::devtools::protocol::DOM::Node>* out_root) override;
 
+ private:
+  std::unique_ptr<ui::devtools::protocol::DOM::Node> BuildInitialTree();
+
   ash::WmShell* shell_;
 
   DISALLOW_COPY_AND_ASSIGN(AshDevToolsDOMAgent);
 };
 
 }  // namespace devtools
 }  // namespace ash
 
 #endif  // ASH_COMMON_DEVTOOLS_ASH_DEVTOOLS_DOM_AGENT_H_