Add an accessor to HeadlessBrowser for the IO thread

headless/public/headless_browser.h

Index: headless/public/headless_browser.h
diff --git a/headless/public/headless_browser.h b/headless/public/headless_browser.h
index 6c7dfe4cefa607c2eca9c8c6054ec5c37dea9871..954b79e0db4609324864c8ef209bbf7db28be0d8 100644
--- a/headless/public/headless_browser.h
+++ b/headless/public/headless_browser.h
@@ -55,14 +55,18 @@ class HEADLESS_EXPORT HeadlessBrowser {
   virtual HeadlessBrowserContext* GetBrowserContextForId(
       const std::string& id) = 0;
 
-  // Returns a task runner for submitting work to the browser main thread.
-  virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserMainThread()
-      const = 0;
-
   // Returns a task runner for submitting work to the browser file thread.
   virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserFileThread()
       const = 0;
 
+  // Returns a task runner for submitting work to the browser io thread.
+  virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserIOThread()
+      const = 0;
+
+  // Returns a task runner for submitting work to the browser main thread.
+  virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserMainThread()
+      const = 0;
+
   // Requests browser to stop as soon as possible. |Run| will return as soon as
   // browser stops.
   // IMPORTANT: All pointers to HeadlessBrowserContexts and HeadlessWebContents