Explicitly make BrowserThread::SetDelegate specific to the IO thread.

content/public/browser/browser_thread_delegate.h

 // Copyright (c) 2012 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 CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_DELEGATE_H_
 #define CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_DELEGATE_H_
 
 namespace content {
 
-// A class with this type may be registered via
-// BrowserThread::SetDelegate.
+// BrowserThread::SetDelegate was deprecated, this is now only used by
+// BrowserThread::SetIOThreadDelegate.
 //
-// If registered as such, it will schedule to run Init() before the
-// message loop begins and the schedule InitAsync() as the first
-// task on its message loop (after the BrowserThread has done its own
-// initialization), and receive a CleanUp call right after the message
-// loop ends (and before the BrowserThread has done its own clean-up).
+// When registered as such, it will schedule to run Init() before the message
+// loop begins and receive a CleanUp call right after the message loop ends (and
+// before the BrowserThread has done its own clean-up).
 class BrowserThreadDelegate {
  public:
-  virtual ~BrowserThreadDelegate() {}
+  virtual ~BrowserThreadDelegate() = default;
 
   // Called prior to starting the message loop
   virtual void Init() = 0;
 
   // Called just after the message loop ends.
   virtual void CleanUp() = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_DELEGATE_H_