Changes

|- All Mochitests running and passing in central (unrelated to session history or other long-term ongoing projects) || Kris Maglione, Andrew McCreight, Kashav Madan

|- All non-fission tests passing in ‘ash’ || Peter Van Der Beken, Olli Pettay

|- All Reftests with fission passing on central || Jonathan Watt

|- WindowProxy transplanting || Andrew McCreight

|- DocumentChannel & Process switching || Matt Woodrow, Jean-Yves Avenard

|- Make history navigations asynchronous || Olli Pettay

|- Screenshot platform API || Matt Woodrow

|- The 'load' event should wait for OOP-iframes to load || Jonathan Watt

|- BrowsingContextGroup actors || Andreas Farre

|- Kickoff: Replace nsIDocShellTreeItem with BrowsingContext || Kannan Vijayan

|- Kickoff: Make session history IPC async || Olli Pettay, Anny Gakhokidze

|- Make it clear that methods which won't work past Fission are process-bound || Kannan Vijayan

|- Convert content-sessionStore to C++ || Alphan Chen

|- Convert SessionHistoryListener to C++ || Alphan Chen

|- Make the audio/video controls Fission-aware || Abdoulaye Oumar Ly

|- Make the fullscreen code Fission-aware || Abdoulaye Oumar Ly

|- Make Plugin Click-to-Play Fission-aware || Gijs Kruitbosch

|- Make 'Find in page' work in fission world || Neil Deakin

|- Make Login Manager fission-compatible || Neil Deakin

|- Make RemotePrompt Fission-compatible || Mike Conley

|- Make DateTimePicker Fission-aware || Abdoulaye Oumar Ly

|- Add browser tests for a11y tree and events on OOP iframes || James Teh