Introduce "browsingContext.setUserAgent" command

Bug: #448

index.bs

@@ -1880,6 +1880,7 @@ BrowsingContextCommand = (
   browsingContext.Navigate //
   browsingContext.Print //
   browsingContext.Reload //
+  browsingContext.SetUserAgent //
   browsingContext.SetViewport //
   browsingContext.TraverseHistory
 )
@@ -6530,7 +6531,7 @@ strong [=/map=] from handle ids to their corresponding objects.
 script.InternalId = text;
 </pre>
 
-The <code>script.InternalId</code> type represents the id of 
+The <code>script.InternalId</code> type represents the id of
 a previously serialized <code>script.RemoteValue</code> during
 [=serialize as a remote value|serialization=].
 
@@ -7526,7 +7527,7 @@ To <dfn>set internal ids if needed</dfn> given |serialization internal map|,
     1. If |previously serialized remote value| does not have a field
        <code>internalId</code>, run the following steps:
 
-      1. Let |internal id| be the string representation of a [[!RFC4122|UUID]] 
+      1. Let |internal id| be the string representation of a [[!RFC4122|UUID]]
          based on truly random, or pseudo-random numbers.
 
       1. Set the <code>internalId</code> field of