Make root layer DCHECK in RWC::layout less general

WebViewImpl always makes a root layer in layout, but not all
RenderWidgets do.  Instead, only DCHECK if there's a readback
request.

R=jamesr@chromium.org
BUG=397321,419644

Review URL: https://codereview.chromium.org/622823002

Cr-Commit-Position: refs/heads/master@{#297880}

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());
   }