Auto merge of #22051 - mandreyel:track-focused-bc-of-each-tlbc, r=cbrewster

Update Constellation to track each browser's focused browsing context

Since there may be multiple browsers (top-level browsing contexts), each has a focused browsing context. However, we were not keeping track of each browser's focused browsing context, so e.g when switching tabs the `Constellation::focused_browsing_context_id` would not be set to the switched-to browser's focused browsing context.

This PR introduces a `browser_ids` `HashMap` in constellation, that maps each of the top-level browsing context's ids to their currently focused browsing context's id, so that when the active browser is changed with the `SelectBrowser` message, we can look up and restore the selected browser's focused browsing context.

This is a wip. For one, I'm not a fan of adding another hash map to constellation, and since there already is a hash map for keeping track of a browser's joint session history, we could introduce some `Browser` struct to hold data for a browser like its session history and focused browsing context (and possibly more later). But wanted to implement the bare-bones logic to first ensure correctness and will refactor later. Also, we may need new tests but I'm not sure.

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