This commit removes nsIRemoteTab as a parent class from BrowserParent,
so that BrowserHost is the only concrete implementation of nsIRemoteTab.
Some static_cast's are updated to cast to BrowserHost, and other places
have to be updated to pass a BrowserHost instead of a BrowserParent.
WindowGlobalParent had a getter to return it's managing BrowserParent
as a nsIRemoteTab. I couldn't find a use of this in-tree, so I've just
opt-ed to remove it. If there's a use-case, we can add something back
in.
Differential Revision: https://phabricator.services.mozilla.com/D31444
eda6ea6e51