Add an accessor to HeadlessBrowser for the IO thread

headless/lib/browser/headless_browser_impl.h

 // Copyright 2015 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 HEADLESS_LIB_BROWSER_HEADLESS_BROWSER_IMPL_H_
 #define HEADLESS_LIB_BROWSER_HEADLESS_BROWSER_IMPL_H_
 
 #include "headless/public/headless_browser.h"
 
 #include <memory>
@@ skipping 20 matching lines @@
 
 class HeadlessBrowserImpl : public HeadlessBrowser {
  public:
   HeadlessBrowserImpl(
       const base::Callback<void(HeadlessBrowser*)>& on_start_callback,
       HeadlessBrowser::Options options);
   ~HeadlessBrowserImpl() override;
 
   // HeadlessBrowser implementation:
   HeadlessBrowserContext::Builder CreateBrowserContextBuilder() override;
+  scoped_refptr<base::SingleThreadTaskRunner> BrowserFileThread()
+      const override;
+  scoped_refptr<base::SingleThreadTaskRunner> BrowserIOThread() const override;
   scoped_refptr<base::SingleThreadTaskRunner> BrowserMainThread()
       const override;
-  scoped_refptr<base::SingleThreadTaskRunner> BrowserFileThread()
-      const override;
 
   void Shutdown() override;
 
   std::vector<HeadlessBrowserContext*> GetAllBrowserContexts() override;
   HeadlessWebContents* GetWebContentsForDevToolsAgentHostId(
       const std::string& devtools_agent_host_id) override;
   HeadlessBrowserContext* GetBrowserContextForId(
       const std::string& id) override;
 
   void set_browser_main_parts(HeadlessBrowserMainParts* browser_main_parts);
@@ skipping 29 matching lines @@
 
   base::WeakPtrFactory<HeadlessBrowserImpl> weak_ptr_factory_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(HeadlessBrowserImpl);
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_LIB_BROWSER_HEADLESS_BROWSER_IMPL_H_