[css-layout-api] Name the FragmentResultOptions argument (w3c#858)

This was invalid Web IDL. Also make it optional since there are no required members.

Author: foolip

css-layout-api/Overview.bs

@@ -1594,7 +1594,7 @@ dictionary FragmentResultOptions {
     BreakTokenOptions breakToken = null;
 };
 
-[Constructor(FragmentResultOptions)]
+[Constructor(optional FragmentResultOptions options)]
 interface FragmentResult {
     readonly attribute double inlineSize;
     readonly attribute double blockSize;