Convert DnsTransactionTest to use SequencedSocketData

net/socket/socket_test_util.h

Index: net/socket/socket_test_util.h
diff --git a/net/socket/socket_test_util.h b/net/socket/socket_test_util.h
index 3bd88d2772f751a822613c856d5e2c2f98da90e1..0858b6ea8106eea0721f667197c81e63328549e9 100644
--- a/net/socket/socket_test_util.h
+++ b/net/socket/socket_test_util.h
@@ -500,6 +500,10 @@ class SequencedSocketData : public SocketDataProvider {
   MockWriteResult OnWrite(const std::string& data) override;
   void Reset() override;
 
+  // Returns true if any data was written to the socket, even if the write
+  // has not completed.
+  bool WasDataWritten() const;
+
   // Returns true if all data has been read.
   bool at_read_eof() const;