[iOS] Fix ConsentBump presentation style

The presentation style should be UIModalPresentationFormSheet.

Bug: none
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I2e2e6c20b940e2c455a7605b2bdfc874ec2e9f4e
Reviewed-on: https://chromium-review.googlesource.com/1183670Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584971}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -5059,7 +5059,7 @@ applicationCommandEndpoint:(id<ApplicationCommands>)applicationCommandEndpoint {
   self.consentBumpCoordinator.delegate = self;
   [self.consentBumpCoordinator start];
   self.consentBumpCoordinator.viewController.modalPresentationStyle =
-      UIModalPresentationPageSheet;
+      UIModalPresentationFormSheet;
   [self presentViewController:self.consentBumpCoordinator.viewController
                      animated:YES
                    completion:nil];