Decouple Metrics Initialization from IO Thread Initialization

chrome/browser/io_thread.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 CHROME_BROWSER_IO_THREAD_H_
 #define CHROME_BROWSER_IO_THREAD_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 222 matching lines @@
   // called on the IO thread. If |host_filter| is not null, only hosts matched
   // by it are deleted from the cache.
   void ClearHostCache(
       const base::Callback<bool(const std::string&)>& host_filter);
 
   const net::HttpNetworkSession::Params& NetworkSessionParams() const;
 
   base::TimeTicks creation_time() const;
 
   // Returns the callback for updating data use prefs.
-  const metrics::UpdateUsagePrefCallbackType& GetMetricsDataUseForwarder();
+  metrics::UpdateUsagePrefCallbackType GetMetricsDataUseForwarder();
 
   // Registers the |observer| for new STH notifications.
   void RegisterSTHObserver(net::ct::STHObserver* observer);
 
   // Un-registers the |observer|.
   void UnregisterSTHObserver(net::ct::STHObserver* observer);
 
   // Returns true if the indicated proxy resolution features are
   // enabled. These features are controlled through
   // preferences/policy/commandline.
@@ skipping 139 matching lines @@
       system_url_request_context_getter_;
 
   // True if QUIC is allowed by policy.
   bool is_quic_allowed_by_policy_;
 
   // True if HTTP/0.9 is allowed on non-default ports by policy.
   bool http_09_on_non_default_ports_enabled_;
 
   const base::TimeTicks creation_time_;
 
-  // Callback for updating data use prefs which needs to be initialized on UI
-  // thread and passed to |DataUseNetworkDelegate|.
-  metrics::UpdateUsagePrefCallbackType metrics_data_use_forwarder_;
-
   base::WeakPtrFactory<IOThread> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(IOThread);
 };
 
 #endif  // CHROME_BROWSER_IO_THREAD_H_