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

net/spdy/platform/impl/spdy_string_utils_impl.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_PLATFORM_IMPL_SPDY_STRING_UTILS_IMPL_H_
 #define NET_SPDY_PLATFORM_IMPL_SPDY_STRING_UTILS_IMPL_H_
 
 #include <sstream>
-#include <string>
 
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
+#include "net/spdy/platform/api/spdy_string.h"
 
 namespace net {
 
 template <typename... Args>
-inline std::string SpdyStrCatImpl(const Args&... args) {
+inline SpdyString SpdyStrCatImpl(const Args&... args) {
   std::ostringstream oss;
   int dummy[] = {1, (oss << args, 0)...};
   static_cast<void>(dummy);
   return oss.str();
 }
 
 template <typename... Args>
-inline void SpdyStrAppendImpl(std::string* output, Args... args) {
+inline void SpdyStrAppendImpl(SpdyString* output, Args... args) {
   output->append(SpdyStrCatImpl(args...));
 }
 
 template <typename... Args>
-inline std::string SpdyStringPrintfImpl(const Args&... args) {
+inline SpdyString SpdyStringPrintfImpl(const Args&... args) {
   return base::StringPrintf(std::forward<const Args&>(args)...);
 }
 
 template <typename... Args>
 inline void SpdyStringAppendFImpl(const Args&... args) {
   base::StringAppendF(std::forward<const Args&>(args)...);
 }
 
 inline char SpdyHexDigitToIntImpl(char c) {
   return base::HexDigitToInt(c);
 }
 
 }  // namespace net
 
 #endif  // NET_SPDY_PLATFORM_IMPL_SPDY_STRING_UTILS_IMPL_H_