Make BreakToken#childBreakTokens a FrozenArray (w3c#786)

Fixes w3c#758.

Author: foolip

css-layout-api/Overview.bs

@@ -825,7 +825,7 @@ interface ChildBreakToken {
 
 [Exposed=LayoutWorklet]
 interface BreakToken {
-    readonly attribute sequence<ChildBreakToken> childBreakTokens;
+    readonly attribute FrozenArray<ChildBreakToken> childBreakTokens;
     readonly attribute any data;
 };