Factor out StringAppendF into spdy/platform/.

net/spdy/platform/api/spdy_string_utils.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_API_SPDY_STRING_UTILS_H_
 #define NET_SPDY_PLATFORM_API_SPDY_STRING_UTILS_H_
 
 #include <string>
 #include <utility>
 
 #include "net/spdy/platform/impl/spdy_string_utils_impl.h"
 
 namespace net {
 
 template <typename... Args>
 inline std::string SpdyStrCat(const Args&... args) {
   return SpdyStrCatImpl(std::forward<const Args&>(args)...);
 }
 
 template <typename... Args>
 inline void SpdyStrAppend(std::string* output, const Args&... args) {
   SpdyStrAppendImpl(output, std::forward<const Args&>(args)...);
 }
 
 template <typename... Args>
 inline std::string SpdyStringPrintf(const Args&... args) {
   return SpdyStringPrintfImpl(std::forward<const Args&>(args)...);
 }
 
+template <typename... Args>
+inline void SpdyStringAppendF(const Args&... args) {
+  SpdyStringAppendFImpl(std::forward<const Args&>(args)...);
+}
+
 }  // namespace net
 
 #endif  // NET_SPDY_PLATFORM_API_SPDY_STRING_UTILS_H_