Use fn pipeline_id consistently, not fn pipeline

Consistently use the name 'pipeline_id' to refer to a function that
returns an (optional) PipelineId.

This was prompted by discovering both fn pipeline and fn pipeline_id
doing the same job in htmliframeelement.rs.

Note that there is fn pipeline in components/compositing/compositor.rs,
but that actually returns an Option<&CompositionPipeline>, not any kind
of PipelineId.

components/script/dom/serviceworkercontainer.rs

@@ -100,7 +100,7 @@ impl ServiceWorkerContainerMethods for ServiceWorkerContainer {
                                                                  script_url,
                                                                  scope_str.clone(),
                                                                  self);
-        ScriptThread::set_registration(scope, &*worker_registration, self.global().r().pipeline());
+        ScriptThread::set_registration(scope, &*worker_registration, self.global().r().pipeline_id());
         Ok(worker_registration)
     }
 }