[iOS] Update the sign-in primary button according to view controller state

The primary button needs to be updated according to current state of
ChromeSigninViewController.
Bug introduced with crrev.com/c/1695411

Bug: 987985
Change-Id: Idfbfbaa48b62fead98d23d2ba464fc46c3285e62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720836Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681386}

ios/chrome/browser/ui/authentication/chrome_signin_view_controller.mm

@@ -1028,7 +1028,19 @@ enum AuthenticationState {
       [self updateGradientColors];
       // As of iOS 13 Beta 3, MDCFlatButton doesn't update it's colors
       // automatically. These lines do it instead.
-      [self updatePrimaryButtonForIdentityPickerState];
+      switch (_currentState) {
+        case IDENTITY_PICKER_STATE:
+          [self updatePrimaryButtonForIdentityPickerState];
+          break;
+        case IDENTITY_SELECTED_STATE:
+          [self updatePrimaryButtonForIdentitySelectedState];
+          break;
+        case NULL_STATE:
+        case SIGNIN_PENDING_STATE:
+        case DONE_STATE:
+          // Transitional states. No need to updated the primary button.
+          break;
+      }
       [self setSecondaryButtonStyling:_secondaryButton];
     }
   }