[multiball] Trigger UI teardown on legacy startup through scene state.

Trigger teardownUI with legacy startup by updating the main scene state
instead of direct call in MainController.

Bug: 1045660
Change-Id: I6ca5e3d5c9dfd5b732da64f95eabdedff0571a15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2231119
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789531}

ios/chrome/app/application_delegate/app_state.mm

@@ -462,6 +462,11 @@ initWithBrowserLauncher:(id<BrowserLauncher>)browserLauncher
         NO;
   }
 
+  // Trigger UI teardown on iOS 12.
+  if (!IsSceneStartupSupported()) {
+    self.mainSceneState.activationLevel = SceneActivationLevelUnattached;
+  }
+
   [_startupInformation stopChromeMain];
 }
 

ios/chrome/app/main_controller.mm

@@ -699,12 +699,6 @@ void MainControllerAuthenticationServiceDelegate::ClearBrowsingData(
 }
 
 - (void)stopChromeMain {
-  // Teardown UI state that is associated with scenes.
-  for (SceneState* sceneState in self.appState.connectedScenes) {
-    sceneState.activationLevel = SceneActivationLevelUnattached;
-  }
-  // End of per-window code.
-
   OmahaService::Stop();
 
   [_spotlightManager shutdown];