The functionality to compute the buckets has been split in two steps:

1. Guest frame computes and sends a list of the anchors's top/bottom
   positions with the `anchorsChanged` RPC event.

2. In the host frame, `BucketBar` uses the list anchor's positions to
   compute the buckets.

There is one issue with the current approach: if an iframe doesn't cover
the entire host frame's viewport buckets can appear offset.