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/storage.rs

@@ -206,7 +206,7 @@ impl Runnable for StorageEventRunnable {
             assert!(UrlHelper::SameOrigin(&ev_url, &it_window.get_url()));
             // TODO: Such a Document object is not necessarily fully active, but events fired on such
             // objects are ignored by the event loop until the Document becomes fully active again.
-            if ev_window.pipeline() != it_window.pipeline() {
+            if ev_window.pipeline_id() != it_window.pipeline_id() {
                 storage_event.upcast::<Event>().fire(it_window.upcast());
             }
         }