Break the association between pipelines and browsing context actors.
Now there is one browsing context actor per actual browsing context,
and individual actors keep track of known pipelines as necessary.
There is also one console/performance/timeline/inspector/etc. actor
per browsing context.

This also centralizes more information in the browsing context actor.
Rather than duplicating state for the active pipeline in actors that
need to use it, each actor now remembers the name of its associated
browsing context actor and obtains that state whenever it's necessary.