Add convenience HighestPriority and LowestPriority methods to QuicUtils.

net/quic/quic_utils.h

Index: net/quic/quic_utils.h
diff --git a/net/quic/quic_utils.h b/net/quic/quic_utils.h
index 0ee117416336ad30849812fac976c8daf711d305..23f3b53a61179160ad336232321c8d9eff293942 100644
--- a/net/quic/quic_utils.h
+++ b/net/quic/quic_utils.h
@@ -75,6 +75,10 @@ class NET_EXPORT_PRIVATE QuicUtils {
   static char* AsChars(unsigned char* data) {
     return reinterpret_cast<char*>(data);
   }
+
+  static QuicPriority LowestPriority();
+
+  static QuicPriority HighestPriority();
 };
 
 // Utility function that returns an IOVector object wrapped around |str|.