Bug 1667081 - Part 3: Remove windowLowered in nsIFocusManager; r=hsivonen

This patch also does following changes,
- Assert aWindow instead of early return given all caller passes a non-null window.
- Make it returning void given all caller doesn't actaully check the return value.

Differential Revision: https://phabricator.services.mozilla.com/D91289

xpfe/appshell/AppWindow.cpp

@@ -2862,7 +2862,9 @@ void AppWindow::WindowDeactivated() {
   nsCOMPtr<nsPIDOMWindowOuter> window =
       mDocShell ? mDocShell->GetWindow() : nullptr;
   nsFocusManager* fm = nsFocusManager::GetFocusManager();
-  if (fm && window && !fm->IsTestMode()) fm->WindowLowered(window);
+  if (fm && window && !fm->IsTestMode()) {
+    fm->WindowLowered(window);
+  }
 }
 
 #ifdef USE_NATIVE_MENUS