[iOS] Presenting sign-in promo in sheet style

Sign-in promo is always presented in sheet style (user sign-in or
sign-in promo). This patch doesn't change the first run, since it
is not presented.

Bug: 971989
Change-Id: I7923a728d6912013903b9fc71a24fef29f2b9b05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2139764
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757080}

ios/chrome/browser/ui/authentication/signin/user_signin/user_signin_coordinator.mm

@@ -372,6 +372,7 @@ const CGFloat kFadeOutAnimationDuration = 0.16f;
 // controller. First run requires an additional transitional fade animation when
 // presenting this view.
 - (void)presentUserSigninViewController {
+  self.viewController.modalPresentationStyle = UIModalPresentationFormSheet;
   switch (self.signinIntent) {
     case UserSigninIntentFirstRun: {
       // Displays the sign-in screen with transitions specific to first-run.
@@ -413,7 +414,7 @@ const CGFloat kFadeOutAnimationDuration = 0.16f;
     }
     case UserSigninIntentSignin: {
       DCHECK(self.baseViewController);
-      self.viewController.modalPresentationStyle = UIModalPresentationFormSheet;
+
       [self.baseViewController presentViewController:self.viewController
                                             animated:YES
                                           completion:nil];