Decouple Metrics Initialization from IO Thread Initialization

chrome/browser/io_thread.h

Index: chrome/browser/io_thread.h
diff --git a/chrome/browser/io_thread.h b/chrome/browser/io_thread.h
index ab7f5568e05f01a88b76eef457b53c2d6a65e77d..f6586cb4a744a109be61cea242559774683bfde1 100644
--- a/chrome/browser/io_thread.h
+++ b/chrome/browser/io_thread.h
@@ -240,7 +240,7 @@ class IOThread : public content::BrowserThreadDelegate {
   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);
@@ -400,10 +400,6 @@ class IOThread : public content::BrowserThreadDelegate {
 
   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);