4773: Closes #4749: Fix for bug 1522829 r=pocmo a=csadilek

We basically already have the parent session everywhere we need it, we now just forward the corresponding engine session so we can pass it along to GeckoView e.g. when opening a new (regular or private) tab, and when reacting to window requests.

@pocmo one discussion point is whether or not we have to create an engine session for the parent if it's no longer there (e.g. when re-loading a child tab after a restart). I decided **not** to do that as we'd be loading all parent sessions on startup and the child session should already have the right state.

Also, I didn't want to overload the method as named args with default values make this easy enough to read in Kotlin. 

Co-authored-by: Christian Sadilek <christian.sadilek@gmail.com>