Add a new QUIC platform API for text utilities.

net/quic/core/quic_session.cc

Index: net/quic/core/quic_session.cc
diff --git a/net/quic/core/quic_session.cc b/net/quic/core/quic_session.cc
index 29d8de24bb543e9017ad1b460e9fda688bc1c7ec..255c05e0d3e149415ee1950a2b0f3c35e77fec20 100644
--- a/net/quic/core/quic_session.cc
+++ b/net/quic/core/quic_session.cc
@@ -6,7 +6,6 @@
 
 #include "base/memory/ptr_util.h"
 #include "base/stl_util.h"
-#include "base/strings/string_number_conversions.h"
 #include "net/quic/core/crypto/proof_verifier.h"
 #include "net/quic/core/quic_bug_tracker.h"
 #include "net/quic/core/quic_connection.h"
@@ -14,10 +13,8 @@
 #include "net/quic/core/quic_flow_controller.h"
 #include "net/quic/platform/api/quic_str_cat.h"
 
-using base::IntToString;
 using base::StringPiece;
 using std::string;
-using net::SpdyPriority;
 
 namespace net {