Factor out StringAppendF into spdy/platform/.

net/spdy/platform/api/spdy_string_utils.h

Index: net/spdy/platform/api/spdy_string_utils.h
diff --git a/net/spdy/platform/api/spdy_string_utils.h b/net/spdy/platform/api/spdy_string_utils.h
index 852bf698aad61e78394a4ef5a305b541f7904ada..fa23b45966a5d83e70c1092529e00aa0d78d5858 100644
--- a/net/spdy/platform/api/spdy_string_utils.h
+++ b/net/spdy/platform/api/spdy_string_utils.h
@@ -27,6 +27,11 @@ 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_