Add convenience HighestPriority and LowestPriority methods to QuicUtils.

net/quic/quic_data_stream.cc

Index: net/quic/quic_data_stream.cc
diff --git a/net/quic/quic_data_stream.cc b/net/quic/quic_data_stream.cc
index 29616b8c5ab56844006efe16d62bd9fbb883a247..6b03a155199acc273838378aeeabc171715f7e8f 100644
--- a/net/quic/quic_data_stream.cc
+++ b/net/quic/quic_data_stream.cc
@@ -308,7 +308,7 @@ uint32 QuicDataStream::StripPriorityAndHeaderId(
 
       // Spdy priorities are inverted, so the highest numerical value is the
       // lowest legal priority.
-      if (temporary_priority > static_cast<QuicPriority>(kLowestPriority)) {
+      if (temporary_priority > QuicUtils::LowestPriority()) {
         session()->connection()->SendConnectionClose(QUIC_INVALID_PRIORITY);
         return 0;
       }