SubpageId was originally introduced in 2013 to help iframes keep track of
their associated (children) pipelines. However, since each pipeline
already has a PipelineId, and those are unique, those are sufficient
to keep track of children.
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.
This currently breaks Servo on Android, because there are a number of
interdependent changes that cannot easily land serially in a way that
keeps it working throughout. We expect to fix this in the near future.
This ensures that UncheckedUnwrap(obj, /* stopAtOuter = */ true) will indeed
stop at the WindowProxy ("outer window", in older SpiderMonkey jargon), which
is necessary to prevent raw Window ("inner window") objects becoming visible
to JavaScript code, or tripping assertions that we don't do that.
