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/console.rs

@@ -18,7 +18,7 @@ impl Console {
         if let Some(chan) = global.devtools_chan() {
             let console_message = prepare_message(level, message);
             let devtools_message = ScriptToDevtoolsControlMsg::ConsoleAPI(
-                global.pipeline(),
+                global.pipeline_id(),
                 console_message,
                 global.get_worker_id());
             chan.send(devtools_message).unwrap();