Factor out StringAppendF into spdy/platform/.

net/spdy/platform/impl/spdy_string_utils_impl.h

Index: net/spdy/platform/impl/spdy_string_utils_impl.h
diff --git a/net/spdy/platform/impl/spdy_string_utils_impl.h b/net/spdy/platform/impl/spdy_string_utils_impl.h
index 7f62318b5b18d7fe42362a22bbeefd007748f1e9..367c94d15ad3e79e8a9652b2efce3f7d3729ea5a 100644
--- a/net/spdy/platform/impl/spdy_string_utils_impl.h
+++ b/net/spdy/platform/impl/spdy_string_utils_impl.h
@@ -30,6 +30,11 @@ 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_