Add browsingContext.setViewport (#425)

* Add emulation.setViewport

This provides the ability to customize the viewport dimensions for a top level navigable e.g. for emulating a different device class.

Co-authored-by: Julian Descottes <julian.descottes@gmail.com>
Co-authored-by: jgraham <james@hoppipolla.co.uk>

index.bs

@@ -2729,6 +2729,62 @@ The [=remote end steps=] with |command parameters| are:
 
 </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.
+
+<dl>
+   <dt>Command Type</dt>
+   <dd>
+    <pre class="cddl remote-cddl">
+      browsingContext.SetViewport = {
+        method: "browsingContext.setViewport",
+        params: browsingContext.SetViewportParameters
+      };
+
+      browsingContext.SetViewportParameters = {
+        context: browsingContext.BrowsingContext,
+        viewport: browsingContext.Viewport / null,
+      }
+
+      browsingContext.Viewport = {
+        width: js-uint,
+        height: js-uint,
+      }
+    </pre>
+   </dd>
+   <dt>Result Type</dt>
+   <dd>
+    <pre class="cddl">
+     EmptyResult
+    </pre>
+   </dd>
+</dl>
+
+<div algorithm="remote end steps for browsingContext.setViewport">
+
+The [=remote end steps=] with |command parameters| are:
+
+1. Let |context id| be the value of the |command parameters|["<code>context</code>"] field.
+
+1. Let |context| be the result of [=trying=] to [=get a browsing context=] with |context id|.
+
+1. If |context| is not a [=top-level browsing context=], return [=error=] with [=error code=] [=invalid argument=].
+
+1. Let |viewport parameter| be the |command parameters|["<code>viewport</code>"] field.
+
+1. If |viewport parameter| is not null:
+    1. Let |width| be the |viewport parameter|["<code>width</code>"]
+    1. Let |height| be the |viewport parameter|["<code>height</code>"]
+    1. If the |width| or |height| is outside of the implementation-specific or runtime constraints, return an [=error=] with the [=error code=] [=unsupported operation=].
+    1. Set the layout [[css2#viewport]] of the |context| to be |width| CSS pixels wide and |height| CSS pixels high.
+1. Else:
+    1. Set the layout [[css2#viewport]] of |context| to the implementation-defined default layout viewport for |context|.
+1. Run the [[cssom-view-1#resizing-viewports]] steps.
+1. Return [=success=] with data null.
+
+</div>
+
 
 ### Events ### {#module-contexts-events}