Include quic_str_cat.h in all the places it's used.

net/quic/core/quic_spdy_session.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/core/quic_spdy_session.h"
 
 #include <utility>
 
 #include "net/quic/core/quic_bug_tracker.h"
+#include "net/quic/core/quic_flags.h"
 #include "net/quic/core/quic_headers_stream.h"
 #include "net/quic/platform/api/quic_str_cat.h"
 
 using base::StringPiece;
 using std::string;
 
 namespace net {
 
 namespace {
 
@@ skipping 716 matching lines @@
       set_max_uncompressed_header_bytes);
 }
 
 void QuicSpdySession::CloseConnectionWithDetails(QuicErrorCode error,
                                                  const string& details) {
   connection()->CloseConnection(
       error, details, ConnectionCloseBehavior::SEND_CONNECTION_CLOSE_PACKET);
 }
 
 }  // namespace net