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..ba51d048dd5fc9cde2ae1b89fd5068c7c611e6ba 100644
--- a/chrome/browser/io_thread.h
+++ b/chrome/browser/io_thread.h
@@ -16,6 +16,7 @@
 #include <vector>
 
 #include "base/compiler_specific.h"
+#include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
@@ -239,6 +240,9 @@ class IOThread : public content::BrowserThreadDelegate {
 
   base::TimeTicks creation_time() const;
 
+  // Initializes the callback for updating data use prefs.
+  void InitMetricsDataUseForwarder();
+
   // Returns the callback for updating data use prefs.
   const metrics::UpdateUsagePrefCallbackType& GetMetricsDataUseForwarder();
 
@@ -404,6 +408,11 @@ class IOThread : public content::BrowserThreadDelegate {
   // thread and passed to |DataUseNetworkDelegate|.
   metrics::UpdateUsagePrefCallbackType metrics_data_use_forwarder_;
 
+#if DCHECK_IS_ON()
+  // True if |metrics_data_use_forwarder_| has been set.
+  bool metrics_data_use_forwarder_set_ = false;
+#endif
+
   base::WeakPtrFactory<IOThread> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(IOThread);