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.
