Adds DevTools commands for forced viewport override.

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 82e555b9d7e8d5d0914c39327709519759033985..3f9b7642ffb7f2ffe0652eae420c34e959322bd1 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -709,6 +709,10 @@ void RenderWidgetCompositor::setViewportSize(
   layer_tree_host_->GetLayerTree()->SetViewportSize(device_viewport_size);
 }
 
+WebSize RenderWidgetCompositor::getViewportSize() const {
+  return layer_tree_host_->GetLayerTree()->device_viewport_size();
+}
+
 WebFloatPoint RenderWidgetCompositor::adjustEventPointForPinchZoom(
     const WebFloatPoint& point) const {
   return point;