Decouple Metrics Initialization from IO Thread Initialization

components/metrics/metrics_service.h

Index: components/metrics/metrics_service.h
diff --git a/components/metrics/metrics_service.h b/components/metrics/metrics_service.h
index 3f19a5b34908cc7400440e78062200edc524edd7..ae7729a2aa266f03fba00a41cec8f9ee708829f5 100644
--- a/components/metrics/metrics_service.h
+++ b/components/metrics/metrics_service.h
@@ -206,9 +206,10 @@ class MetricsService : public base::HistogramFlattener {
   // Pushes a log that has been generated by an external component.
   void PushExternalLog(const std::string& log);
 
-  // Returns a callback to data use pref updating function which can be called
-  // from any thread, but this function should be called on UI thread.
-  UpdateUsagePrefCallbackType GetDataUseForwardingCallback();
+  // Updates data usage tracking prefs with the specified values.
+  void UpdateMetricsUsagePrefs(const std::string& service_name,
+                               int message_size,
+                               bool is_cellular);
 
   // Merge any data from metrics providers into the global StatisticsRecorder.
   void MergeHistogramDeltas();