[iOS] Updating the button title case for the sign-in screen

Changing the title case to be:
 + "Cancel"
 + "Yes, I'm In"
 + "No Thanks"
 + "Add Account"
Instead being full upper case. The "More" button already have the right
case.

More button:
https://drive.google.com/open?id=15vqLmU2mCpjz41bOrKdzYvPHubbj2m_y

Before:
https://drive.google.com/open?id=1LYIyLOwRUyiD5Syzvl648cycq42trYVv
https://drive.google.com/open?id=12jBAY5JTTKUgfBwCREWpRnc620btKx-2

After:
https://drive.google.com/open?id=10TwDZEc7_qfapCmb651UHjQijhIX81_C
https://drive.google.com/open?id=1nnnFpYS5JkzhSibXePGf9JUNeue3dnl8

Bug: 971989
Change-Id: Ib60b1a910d669f03c90613e865d7a986351f4230
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142293Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757831}

ios/chrome/app/strings/ios_strings.grd

@@ -196,10 +196,10 @@ locale.  The strings in this file are specific to iOS.
         You can always choose what to sync in <ph name="BEGIN_LINK">BEGIN_LINK</ph>settings<ph name="END_LINK">END_LINK</ph>.
       </message>
       <message name="IDS_IOS_ACCOUNT_UNIFIED_CONSENT_ADD_ACCOUNT" desc="Title of the button to add an account [Length: 15m] [iOS only]">
-        ADD ACCOUNT
+        Add Account
       </message>
       <message name="IDS_IOS_ACCOUNT_UNIFIED_CONSENT_OK_BUTTON" desc="Title of the button to validate the user consent [Length: 15m] [iOS only]">
-        YES, I'M IN
+        Yes, I'm In
       </message>
       <message name="IDS_IOS_ADD_CREDIT_CARD_INVALID_CARD_NUMBER_ALERT" desc="The title of the alert view displaying invalid card number when a user enters invalid card number for the new credit card to save. [iOS only]">
           Invalid Card Number
@@ -771,7 +771,7 @@ Because your account is managed by <ph name="HOSTED_DOMAIN">$1<ex>google.com</ex
         Call
       </message>
       <message name="IDS_IOS_FIRSTRUN_ACCOUNT_CONSISTENCY_SKIP_BUTTON" desc="Title of the button to skip the selection of an account on First Run when account consistency is enabled. [Length: 10em] [iOS only]">
-        No thanks
+        No Thanks
       </message>
       <message name="IDS_IOS_FIRSTRUN_OPT_IN_ACCEPT_BUTTON" desc="User metrics opt-in acceptance button label [iOS only]">
         Accept &amp; Continue

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

@@ -406,7 +406,7 @@ enum AuthenticationButtonType {
   DCHECK(self.unifiedConsentViewController);
   self.skipSigninButton = [[UIButton alloc] init];
   [self addSubviewWithButton:self.skipSigninButton];
-  [self.skipSigninButton setTitle:self.skipSigninButtonTitle.uppercaseString
+  [self.skipSigninButton setTitle:self.skipSigninButtonTitle
                          forState:UIControlStateNormal];
   [self setSkipSigninStylingWithButton:self.skipSigninButton];
   [self.skipSigninButton addTarget:self