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
 )
@@ -3406,6 +3407,50 @@ The [=remote end steps=] with |command parameters| are:
 
 </div>
 
+#### The browsingContext.setUserAgent Command ####  {#command-browsingContext-setUserAgent}
+
+The <dfn export for=commands>browsingContext.setUserAgent</dfn> command sets the
+user agent string for a browsing context.
+
+<dl>
+   <dt>Command Type</dt>
+   <dd>
+    <pre class="cddl remote-cddl">
+      browsingContext.SetUserAgent = (
+        method: "browsingContext.setUserAgent",
+        params: browsingContext.SetUserAgentParameters
+      )
+
+      browsingContext.SetUserAgentParameters = {
+        context: browsingContext.BrowsingContext,
+        userAgent: text / null,
+      }
+    </pre>
+   </dd>
+   <dt>Result Type</dt>
+   <dd>
+    <pre class="cddl">
+     EmptyResult
+    </pre>
+   </dd>
+</dl>
+
+<div algorithm="remote end steps for browsingContext.setUserAgent">
+
+The [=remote end steps=] with |command parameters| are:
+
+1. Let |context id| be the value of the <code>context</code> field of |command
+   parameters|.
+
+1. Let |context| be the result of [=trying=] to [=get a browsing context=] with
+   |context id|.
+
+TODO: Set user agent.
+
+1. Return [=success=] with data null.
+
+</div>
+
 #### The browsingContext.setViewport Command ####  {#command-browsingContext-setViewport}
 
 The <dfn export for=commands>browsingContext.setViewport</dfn> command  specific viewport characteristics (e.g. viewport width and viewport height) on the given top-level browsing context.
@@ -6530,7 +6575,7 @@ 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 +7571,7 @@ 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