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/htmlformelement.rs

@@ -435,7 +435,7 @@ impl HTMLFormElement {
         // Step 2
         let nav = box PlannedNavigation {
             load_data: load_data,
-            pipeline_id: window.pipeline(),
+            pipeline_id: window.pipeline_id(),
             script_chan: window.main_thread_script_chan().clone(),
             generation_id: self.generation_id.get(),
             form: Trusted::new(self)