[css-layout-api-1] Align with Web IDL specification (w3c#964)

* [css-layout-api-1] Align with Web IDL specification

* [css-layout-api-1] Align with Web IDL specification

* expose to LayoutWorklet, not Window

Co-authored-by: Tab Atkins Jr <jackalmage@gmail.com>

Author: autokagami

css-layout-api/Overview.bs

@@ -1587,8 +1587,9 @@ dictionary FragmentResultOptions {
     BreakTokenOptions breakToken = null;
 };
 
-[Constructor(optional FragmentResultOptions options)]
+[Exposed=LayoutWorklet]
 interface FragmentResult {
+    constructor(optional FragmentResultOptions options = {});
     readonly attribute double inlineSize;
     readonly attribute double blockSize;
 };