[iOS] Fix StatusBar appearance when switching mode

This CL makes sure that the status bar appearance is correctly updated
when switching mode (icognito/normal) through opening a new tab.
The status bar theme is chosen based on the
childViewControllerForStatusBarStyle property of the TabGrid.
The notification of UIKit is done at the same time as the child view
controller is updated.

Fixed: 1122239
Change-Id: I847c4d1d70148cabbd824e4bdb40b7aa6fd50847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2395735Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804851}

ios/chrome/browser/ui/main/bvc_container_view_controller.mm

@@ -43,10 +43,6 @@
   [self.view addSubview:bvc.view];
   [bvc didMoveToParentViewController:self];
 
-  // Let the system know that the child has changed so appearance updates can
-  // be made.
-  [self setNeedsStatusBarAppearanceUpdate];
-
   if (IsThumbStripEnabled()) {
     // The background needs to be clear to allow the thumb strip to be seen
     // during the enter/exit thumb strip animation.

ios/chrome/browser/ui/tab_grid/tab_grid_coordinator.mm

@@ -216,6 +216,9 @@
   // container.
   if (self.bvcContainer) {
     self.bvcContainer.currentBVC = viewController;
+    self.baseViewController.childViewControllerForStatusBarStyle =
+        viewController;
+    [self.baseViewController setNeedsStatusBarAppearanceUpdate];
     if (completion) {
       completion();
     }