[iOS] Move access to |statusBarOrientation| to the main thread.

Error found because XCode 9 now runs apps with a
"Main Thread Checker" that detects invalid use of UIKit from a background thread.

Bug: None
Change-Id: I16b159b9dcb0710a37faa66c773063e5b97ae84f
Reviewed-on: https://chromium-review.googlesource.com/723462Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Jean-François Geyelin <jif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509385}

ios/chrome/browser/ui/qr_scanner/camera_controller.mm

@@ -247,8 +247,8 @@
 
     [previewLayer setSession:_captureSession];
     [previewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
-    [self resetVideoOrientation:previewLayer];
     dispatch_async(dispatch_get_main_queue(), ^{
+      [self resetVideoOrientation:previewLayer];
       [_delegate captureSessionIsConnected];
       [self startRecording];
     });