Update browsingContext.traverseHistory

Restrict the command only for top-level browsing contexts.

index.bs

@@ -4096,6 +4096,9 @@ The [=remote end steps=] with |command parameters| are:
 1. Let |navigable| be the result of [=trying=] to [=get a navigable=]
    with |command parameters|["<code>context</code>"].
 
+1. If |navigable| is not a [=/top-level traversable=], return [=error=] with
+   [=error code=] [=invalid argument=].
+
 1. Assert: |navigable| is not null.
 
 1. Let |delta| be |command parameters|["<code>delta</code>"].