[iOS] Advanced sign-in view controller presented as a sheet

Before:
https://drive.google.com/open?id=1GKDMouROHxUjhHsyQ-c-ys8N4HxAgykk

After:
https://drive.google.com/open?id=10y3l1arRrSAxFphOnsHAuEZh9Z8Oo4tW

Bug: 950993
Change-Id: I5996c54a1cacca87d06e55269ead1012b74e4aa9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1564154Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649833}

ios/chrome/browser/ui/settings/google_services/advanced_signin_settings_coordinator.mm

@@ -46,6 +46,8 @@ using l10n_util::GetNSString;
 - (void)start {
   self.advancedSigninSettingsNavigationController =
       [[AdvancedSigninSettingsNavigationController alloc] init];
+  self.advancedSigninSettingsNavigationController.modalPresentationStyle =
+      UIModalPresentationFormSheet;
   self.googleServicesSettingsCoordinator = [[GoogleServicesSettingsCoordinator
       alloc]
       initWithBaseViewController:self.advancedSigninSettingsNavigationController