Factor out StringAppendF into spdy/platform/.

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>
 
@@ skipping 12 matching lines @@
 template <typename... Args>
 inline void SpdyStrAppendImpl(std::string* output, Args... args) {
   output->append(SpdyStrCatImpl(args...));
 }
 
 template <typename... Args>
 inline std::string 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)...);
+}
+
 }  // namespace net
 
 #endif  // NET_SPDY_PLATFORM_IMPL_SPDY_STRING_UTILS_IMPL_H_