window.open does not give correct window/opener access based on browsing contenxt

[bridiver]

The webcontents are created correctly, but electron runs each webview in its own process which breaks the connection. I believe the fix is to add support for guest view into libchromium content and electron to handle the swappedout:// condition. There could also potentially be changes to how/when electron creates new render processes to match the process-per-site-instance model https://www.chromium.org/developers/design-documents/process-models

[bridiver]

partially fixed by brave/muon#2
still need to go back and remove the browserproxy stuff

[bridiver]

brave/muon#3