Make root layer DCHECK in RWC::layout less general

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 c8f39fce32e49443b30e3f7703453d42da3f9266..eee9a2ecacd7c8fda6321d52f9144435eb0434ce 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -786,8 +786,8 @@ void RenderWidgetCompositor::BeginMainFrame(const cc::BeginFrameArgs& args) {
 void RenderWidgetCompositor::Layout() {
   widget_->webwidget()->layout();
 
-  DCHECK(layer_tree_host_->root_layer());
   if (temporary_copy_output_request_) {
+    DCHECK(layer_tree_host_->root_layer());
     layer_tree_host_->root_layer()->RequestCopyOfOutput(
         temporary_copy_output_request_.Pass());
   }