Auto merge of #703 - jrmuizel:root-pipeline2, r=kvark

Avoid frame notification when root pipeline doesn't exist

If we don't have a pipeline to draw don't try to draw it.

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webrender/src/render_backend.rs

@@ -214,9 +214,13 @@ impl RenderBackend {
                             }
                         }
                         ApiMsg::SetRootPipeline(pipeline_id) => {
-                            let frame = profile_counters.total_time.profile(|| {
-                                self.scene.set_root_pipeline_id(pipeline_id);
+                            self.scene.set_root_pipeline_id(pipeline_id);
+
+                            if self.scene.display_lists.get(&pipeline_id).is_none() {
+                                continue;
+                            }
 
+                            let frame = profile_counters.total_time.profile(|| {
                                 self.build_scene();
                                 self.render()
                             });