[iOS] Fix NTP layout issue during animations

This CL makes sure that the NTP is correctly layout with MultiWindow.
With MultiWindow, the NTP view is created with a frame of CGRectZero.
Without MultiWindow, the frame is the one of the app.
Forcing the layout after setting the frame, ensures that the elements
are correctly positioned and sized.

Fixed: 1121993
Change-Id: I1c8ae1c468934a36eedd7687348547bf5885e1e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410379
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806670}

ios/chrome/browser/ui/browser_view/browser_view_controller.mm

@@ -2325,6 +2325,7 @@ NSString* const kBrowserViewControllerSnackbarCategory =
       UIViewController* viewController =
           _ntpCoordinatorsForWebStates[webState].viewController;
       viewController.view.frame = [self ntpFrameForWebState:webState];
+      [viewController.view layoutIfNeeded];
       // TODO(crbug.com/873729): For a newly created WebState, the session will
       // not be restored until LoadIfNecessary call. Remove when fixed.
       webState->GetNavigationManager()->LoadIfNecessary();
@@ -4410,6 +4411,7 @@ NSString* const kBrowserViewControllerSnackbarCategory =
     newPage = [self viewForWebState:webState];
     newPage.userInteractionEnabled = NO;
     newPage.frame = self.view.bounds;
+    [newPage layoutIfNeeded];
   } else {
     [self viewForWebState:webState].frame = self.contentArea.bounds;
     // Setting the frame here doesn't trigger a layout pass. Trigger it manually