Bug 1627684 - Propagate the affect-private-session-lifetime flag for window.open(), r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D71807

dom/ipc/BrowserChild.cpp

@@ -520,6 +520,7 @@ nsresult BrowserChild::Init(mozIDOMWindowProxy* aParent,
   }
 
   docShell->SetAffectPrivateSessionLifetime(
+      mBrowsingContext->UsePrivateBrowsing() ||
       mChromeFlags & nsIWebBrowserChrome::CHROME_PRIVATE_LIFETIME);
 
 #ifdef DEBUG