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/webdriver_handlers.rs

@@ -140,7 +140,7 @@ pub fn handle_get_frame_id(context: &BrowsingContext,
         }
     };
 
-    let frame_id = window.map(|x| x.map(|x| x.pipeline()));
+    let frame_id = window.map(|x| x.map(|x| x.pipeline_id()));
     reply.send(frame_id).unwrap()
 }