This is another aspect of the work to update DOM IPC naming for fission.

TabId is an identifier for a PBrowser actor. This made sense when the implementations were TabParent/TabChild but now that they are BrowserParent/BrowserChild, it's a bit odd.

It seems like there are two different possible meanings here.

1. An identifier for a PBrowser actor (AKA maximal contiguous subtree of browsers)
2. An identifier for a remote top-level tab (AKA nsIRemoteTab)

(1) is a subset of (2), and the current semantics are (1). However, the frontend currently uses this ID as a (2) via the nsIRemoteTab interface.

We should figure out if we want to disentangle these two concepts and how.