Add convenience HighestPriority and LowestPriority methods to QuicUtils.

net/quic/reliable_quic_stream_test.cc

Index: net/quic/reliable_quic_stream_test.cc
diff --git a/net/quic/reliable_quic_stream_test.cc b/net/quic/reliable_quic_stream_test.cc
index 3d15b77f5a46002933a70f7fe9b54e2df4339538..7e82f533142b294207f5d9205a5da01e46bc05f8 100644
--- a/net/quic/reliable_quic_stream_test.cc
+++ b/net/quic/reliable_quic_stream_test.cc
@@ -49,7 +49,9 @@ class TestStream : public ReliableQuicStream {
     return should_process_data_ ? data_len : 0;
   }
 
-  virtual QuicPriority EffectivePriority() const OVERRIDE { return 0; }
+  virtual QuicPriority EffectivePriority() const OVERRIDE {
+    return QuicUtils::HighestPriority();
+  }
 
   using ReliableQuicStream::WriteOrBufferData;
   using ReliableQuicStream::CloseReadSide;