Fix "get shared id for a node" to use node's browsing context (#445)

index.bs

@@ -5234,13 +5234,13 @@ To get the <dfn>handle for an object</dfn> given |realm|, |ownership type| and
 </div>
 
 <div algorithm>
-To <dfn>get shared id for a node</dfn> given |node|, |realm| and |session|:
+To <dfn>get shared id for a node</dfn> given |node|, and |session|:
 
 1. Let |node| be [=unwrapped=] |node|.
 
 1. If |node| does not implement {{Node}}, return null.
 
-1. Let |browsing context| be [=get the browsing context=] with |realm|.
+1. Let |browsing context| be |node|'s [=node document=]'s [=Document/browsing context=].
 
 1. If |browsing context| is null, return null.
 
@@ -5426,8 +5426,7 @@ an |ownership type|, a |serialization internal map|, a |realm| and a |session|:
 
     <dt>|value| is a [=platform object=] that implements {{Node}}
     <dd>
-      1. Let |shared id| be [=get shared id for a node=] with |value|, |realm| and
-         |session|.
+      1. Let |shared id| be [=get shared id for a node=] with |value| and |session|.
 
       1. Let |remote value| be a [=/map=] matching the <code>script.NodeRemoteValue</code>
          production in the [=local end definition=], with the <code>sharedId</code>