Add external data use observer

components/data_usage/core/data_use_aggregator.cc

Index: components/data_usage/core/data_use_aggregator.cc
diff --git a/components/data_usage/core/data_use_aggregator.cc b/components/data_usage/core/data_use_aggregator.cc
index 7613a0126158fe3a49aebe7b4477634c03f6de30..2a4b1c66379e4abe498bf0e385a54c8d0e18a823 100644
--- a/components/data_usage/core/data_use_aggregator.cc
+++ b/components/data_usage/core/data_use_aggregator.cc
@@ -48,6 +48,11 @@ void DataUseAggregator::ReportOffTheRecordDataUse(int64_t tx_bytes,
   // data usage calculations if applicable.
 }
 
+void DataUseAggregator::PassExternalObserver(
+    scoped_ptr<Observer> external_observer) {
+  external_observer_ = external_observer.Pass();
+}
+
 void DataUseAggregator::NotifyDataUse(
     const std::vector<DataUse>& data_use_sequence) {
   DCHECK(thread_checker_.CalledOnValidThread());