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..0503cf37dc608250651ec0a77fec2bd167b99fce 100644
--- a/components/metrics/metrics_service.h
+++ b/components/metrics/metrics_service.h
@@ -207,7 +207,8 @@ class MetricsService : public base::HistogramFlattener {
   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.
+  // from any thread, but this function should be called on UI thread. Can be a
+  // null callback.
   UpdateUsagePrefCallbackType GetDataUseForwardingCallback();
 
   // Merge any data from metrics providers into the global StatisticsRecorder.