[iOS][Alert] Use a prefered font for a11y

Changes the font of the text fields to a preffered font in order to
support dynamic type fonts.

Bug: 959764, 951300
Change-Id: I785a5c577d69e33018a7de5403a0e96751b3d455
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611601
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Auto-Submit: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659488}

ios/chrome/browser/ui/alert_view_controller/alert_view_controller.mm

@@ -314,6 +314,8 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
           textFieldConfiguration.accessibilityIdentifier;
       textField.translatesAutoresizingMaskIntoConstraints = NO;
       textField.delegate = self;
+      textField.font = [UIFont preferredFontForTextStyle:UIFontTextStyleBody];
+
       [fieldStack addArrangedSubview:textField];
       ChromeDirectionalEdgeInsets fieldInsets = ChromeDirectionalEdgeInsetsMake(
           0.0, kTextfieldInset, 0.0, kTextfieldInset);