Skip base::string16 if not necessary for WebString <-> ASCII conversion

content/child/simple_webmimeregistry_impl.cc

Index: content/child/simple_webmimeregistry_impl.cc
diff --git a/content/child/simple_webmimeregistry_impl.cc b/content/child/simple_webmimeregistry_impl.cc
index a2ce6281dac867c62809eff26143c68f0217fd87..4603407b72c75fb8239b99edd7c0c3b50b3d6abb 100644
--- a/content/child/simple_webmimeregistry_impl.cc
+++ b/content/child/simple_webmimeregistry_impl.cc
@@ -22,9 +22,7 @@ namespace content {
 
 // static
 std::string SimpleWebMimeRegistryImpl::ToASCIIOrEmpty(const WebString& string) {
-  return base::IsStringASCII(string)
-      ? base::UTF16ToASCII(base::StringPiece16(string))
-      : std::string();
+  return string.containsOnlyASCII() ? string.ascii() : std::string();
 }
 
 WebMimeRegistry::SupportsType SimpleWebMimeRegistryImpl::supportsMIMEType(