[ios] Correct autoresizingMasks in launch screen incognito blocker.

Bug: 877195
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I6b94acbe327886c90b640b149099db62c17c7719
Reviewed-on: https://chromium-review.googlesource.com/1188609
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585929}

ios/chrome/app/application_delegate/app_state.mm

@@ -205,6 +205,10 @@ initWithBrowserLauncher:(id<BrowserLauncher>)browserLauncher
       UIViewController* launchScreenController =
           base::mac::ObjCCastStrict<UIViewController>([topObjects lastObject]);
       [_incognitoBlocker addSubview:[launchScreenController view]];
+      [launchScreenController view].autoresizingMask =
+          UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
+      _incognitoBlocker.autoresizingMask =
+          UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
     } else {
       InstallBackgroundInView(_incognitoBlocker);
     }