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/browsingcontext.rs

@@ -152,7 +152,7 @@ impl BrowsingContext {
         old
     }
 
-    pub fn pipeline(&self) -> PipelineId {
+    pub fn pipeline_id(&self) -> PipelineId {
         self.id
     }