[resize-observer-1] attributes cannot be sequence<T> w3c#4683 (w3c#4909)

Fix invalid WebIDL by changing sequence to FrozenArray for lastReportedSizes in the ResizeObservation interface. Currently there are still normative references to this non-normative interface, discussed in w3c#3839. These will be addressed in a follow-up.

Author: sahirv

resize-observer-1/Overview.bs

@@ -309,7 +309,7 @@ interface ResizeObservation {
     constructor(Element target);
     readonly attribute Element target;
     readonly attribute ResizeObserverBoxOptions observedBox;
-    readonly attribute sequence<ResizeObserverSize> lastReportedSizes;
+    readonly attribute FrozenArray<ResizeObserverSize> lastReportedSizes;
 };
 </pre>
 <div dfn-type="attribute" dfn-for="ResizeObservation">
@@ -318,7 +318,7 @@ interface ResizeObservation {
     : <dfn>observedBox</dfn>
     :: Which box is being observed.
     : <dfn>lastReportedSizes</dfn>
-    :: Ordered sequence of last reported sizes.
+    :: Ordered array of last reported sizes.
 </div>
 <div dfn-type="method" dfn-for="ResizeObservation">
     : <dfn constructor lt="ResizeObservation(target, options)">new ResizeObservation(target, observedBox)</dfn>