For 2085047 - Drop `BrowsingModeManager` in favor of `AppState` #409 (firefox-autoland@autoland)

Warnings
  • Checking...
Blockers
  • Checking...
Branches browsing_mode_to_appstate -> autoland
Repo firefox-autoland@autoland (git) (git://github.com/Mugurell/firefox.git)
Author Mugurell
State open
Commit Title

For 2085047 - Drop `BrowsingModeManager` in favor of `AppState`

Commit Body

Dropped `BrowsingModeManager` in favor of `AppState` for regular tabs. And used `content.private` from `BrowserState` for custom tabs as they can be used at the same time with the regular tabs (in multiwindow) and could need a different browsing mode than `AppState.mode` value. The approach taken in this PR, edgecases and other potential approaches are documented [here](https://docs.google.com/document/d/1mWz-TOKUE3vsdKif3IBd4_XGjlaFHbSLcykpOTMZaZQ/edit?usp=sharing).

Landings

Not yet Landed
There has been no attempt to land revisions in this stack.