Add external data use observer

components/data_usage/core/data_use_aggregator.h

Index: components/data_usage/core/data_use_aggregator.h
diff --git a/components/data_usage/core/data_use_aggregator.h b/components/data_usage/core/data_use_aggregator.h
index 4c14ec1c3d3b38273096d94fc940b563d2ef1834..ad9a41d5b557d5dcf3389c967398ff20ebb79456 100644
--- a/components/data_usage/core/data_use_aggregator.h
+++ b/components/data_usage/core/data_use_aggregator.h
@@ -50,12 +50,18 @@ class DataUseAggregator {
   // Virtual for testing.
   virtual void ReportOffTheRecordDataUse(int64_t tx_bytes, int64_t rx_bytes);
 
+  // Passes the ownership of an external observer to DataUseAggregator.
+  void PassExternalObserver(scoped_ptr<Observer> external_observer);
+
  private:
   void NotifyDataUse(const std::vector<DataUse>& data_use_sequence);
 
   base::ThreadChecker thread_checker_;
   base::ObserverList<Observer> observer_list_;
 
+  // An observer which is external to Chromium.
+  scoped_ptr<Observer> external_observer_;

[sclittle, 2015/10/09 22:59:28]

Making the DataUseAggregator own one of its observers actually gets quite weird,
especially around destruction time. The Observer's destructor (which calls
data_use_aggregator_->RemoveObserver) will run after the DataUseAggregator's
destructor, which will probably explode.

[tbansal1, 2015/10/12 16:47:36]

changed the owner to IO thread globals.

+
   DISALLOW_COPY_AND_ASSIGN(DataUseAggregator);
 };