[multiball] Group up per-window shutdown code

Groups all per-window shutdown code together and moves it before the
other shutdown code to match expected behaviour of shutdown in
multiwindow world.

Bug: none
Change-Id: I7abf63beecd3d2d6c0479da9c9a6f31477f3ae53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084595
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746364}

ios/chrome/app/main_controller.mm

@@ -776,12 +776,21 @@ - (void)activateFirstUserActionRecorderWithBackgroundTime:
 }
 
 - (void)stopChromeMain {
+  // This code is per-window.
+
   // Teardown UI state that is associated with scenes.
   [self.sceneController teardownUI];
 
   [_mainCoordinator stop];
   _mainCoordinator = nil;
 
+  // Invariant: The UI is stopped before the model is shutdown.
+  DCHECK(!_mainCoordinator);
+  [self.browserViewWrangler shutdown];
+  self.browserViewWrangler = nil;
+
+  // End of per-window code.
+
   [_spotlightManager shutdown];
   _spotlightManager = nil;
 
@@ -796,11 +805,6 @@ - (void)stopChromeMain {
             self.mainBrowserState));
   }
 
-  // Invariant: The UI is stopped before the model is shutdown.
-  DCHECK(!_mainCoordinator);
-  [self.browserViewWrangler shutdown];
-  self.browserViewWrangler = nil;
-
   _extensionSearchEngineDataUpdater = nullptr;
 
   ios::GetChromeBrowserProvider()