Move net-export thread-hopping code into NetLogFileWriter and add IO polled data.

net/log/write_to_file_net_log_observer.h

Index: net/log/write_to_file_net_log_observer.h
diff --git a/net/log/write_to_file_net_log_observer.h b/net/log/write_to_file_net_log_observer.h
index 2ef4189f2ff99ad60033a49fb1b88bfeb3fb791e..87671a2e441b657c19977ab5368a4611f9d09b85 100644
--- a/net/log/write_to_file_net_log_observer.h
+++ b/net/log/write_to_file_net_log_observer.h
@@ -53,10 +53,9 @@ class NET_EXPORT WriteToFileNetLogObserver : public NetLog::ThreadSafeObserver {
   // Stops observing net_log().  Must already be watching.  Must be called
   // before destruction of the WriteToFileNetLogObserver and the NetLog.
   //
-  // |url_request_context| is an optional argument used to added additional
-  // network stack state to the log.  If the context is non-NULL, this must be
-  // called on the context's thread.
-  void StopObserving(URLRequestContext* url_request_context);
+  // |polled_data| is an optional argument used to added additional
+  // network stack state to the log.
+  void StopObserving(base::Value* polled_data);
 
   // net::NetLog::ThreadSafeObserver implementation:
   void OnAddEntry(const NetLogEntry& entry) override;