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 9a797721a07ba2d7258d75accc8c7ac2dcd0dac1..954d8a6f83f8d70f248a7e076514040b735e6e5f 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -700,6 +700,10 @@ 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;