Make BackgroundContentsService start up BackgroundContents with a delay, as for ExtensionHosts.

extensions/browser/extension_host_queue.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 EXTENSIONS_BROWSER_EXTENSION_HOST_QUEUE_H_
 #define EXTENSIONS_BROWSER_EXTENSION_HOST_QUEUE_H_
 
 namespace extensions {
 
-class ExtensionHost;
+class DeferredStartRenderHost;
 
 // An interface for a queue of ExtensionHosts waiting for initialization.
 // This is used to implement different throttling strategies.
 class ExtensionHostQueue {
  public:
   virtual ~ExtensionHostQueue() {}
 
   // Adds a host to the queue for RenderView creation.
-  virtual void Add(ExtensionHost* host) = 0;
+  virtual void Add(DeferredStartRenderHost* host) = 0;
 
   // Removes a host from the queue (for example, it may be deleted before
   // having a chance to start).
-  virtual void Remove(ExtensionHost* host) = 0;
+  virtual void Remove(DeferredStartRenderHost* host) = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_HOST_QUEUE_H_