[ios] Fix startup logo bug iOS 12 iPad-only

There was a bug where the startup logo was not properly
sized in the fake launch screen. Forcing layout fixes
this issue.

Bug: 1014841
Change-Id: Iadfcfb9fb15b01fdca4b7623bdbcc49c42175757
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872450Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Auto-Submit: edchin <edchin@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707854}

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

@@ -1808,6 +1808,8 @@ NSString* const kBrowserViewControllerSnackbarCategory =
       // BVC is contained and its parent bounds to the full screen.
       launchScreenView.frame = self.parentViewController.view.bounds;
       [self.parentViewController.view addSubview:launchScreenView];
+      [launchScreenView setNeedsLayout];
+      [launchScreenView layoutIfNeeded];
 
       // Replace the completion handler sent to the superclass with one which
       // removes |launchScreenView| and resets the status bar. If |completion|