Add SpdyString alias for std::string in net/spdy.

net/spdy/spdy_log_util.h

 // Copyright 2017 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.
 
 #ifndef NET_SPDY_SPDY_LOG_UTIL_H_
 #define NET_SPDY_SPDY_LOG_UTIL_H_
 
 #include <memory>
-#include <string>
 
 #include "base/strings/string_piece.h"
 #include "net/base/net_export.h"
 #include "net/log/net_log_capture_mode.h"
+#include "net/spdy/platform/api/spdy_string.h"
 #include "net/spdy/spdy_header_block.h"
 
 namespace base {
 class ListValue;
 }  // namespace base
 
 namespace net {
 
 // Given an HTTP/2 GOAWAY frame |debug_data|, returns the elided version
 // according to |capture_mode|.
-NET_EXPORT_PRIVATE std::string ElideGoAwayDebugDataForNetLog(
-    NetLogCaptureMode capture_mode,
-    base::StringPiece debug_data);
+NET_EXPORT_PRIVATE SpdyString
+ElideGoAwayDebugDataForNetLog(NetLogCaptureMode capture_mode,
+                              base::StringPiece debug_data);
 
 // Given a SpdyHeaderBlock, return its base::ListValue representation.
 NET_EXPORT_PRIVATE std::unique_ptr<base::ListValue>
 ElideSpdyHeaderBlockForNetLog(const SpdyHeaderBlock& headers,
                               NetLogCaptureMode capture_mode);
 
 }  // namespace net
 
 #endif  // NET_SPDY_SPDY_LOG_UTIL_H_