Change children attribute from sequence<T> to FrozenArray<T> (#795)

Web IDL does not allow attributes to be of type sequence<T>.

Author: foolip

box-tree-api/Overview.bs

@@ -79,7 +79,7 @@ interface DeadFragmentInformation {
 	readonly attribute double top;
 	readonly attribute double left;
 	readonly attribute boolean isOverflowed;
-	readonly attribute sequence<DeadFragmentInformation>? children;
+	readonly attribute FrozenArray<DeadFragmentInformation>? children;
 	readonly attribute DeadFragmentInformation? nextSibling;
 	readonly attribute DeadFragmentInformation? previousSibling;
 	readonly attribute DeadFragmentInformation? nextInBox;