[iOS] Improve readability of UserSigninViewController.

It was difficult to understand the current constraint layout of the
"Turn on Sync" page due to the inconsistencies in the view manipulation.
This patch attempts to make the layout and styling of the views and
buttons more straightforward.

Bug: 1105853
Change-Id: I0c0bca5aa1be5200d3be414804976efc8ed7ef22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2312556
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791778}

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

@@ -219,7 +219,7 @@ const CGFloat kFadeOutAnimationDuration = 0.16f;
 - (void)unifiedConsentCoordinatorNeedPrimaryButtonUpdate:
     (UnifiedConsentCoordinator*)coordinator {
   DCHECK_EQ(self.unifiedConsentCoordinator, coordinator);
-  [self.viewController updatePrimaryButtonStyle];
+  [self.viewController setConfirmationButtonProperties];
 }
 
 #pragma mark - UserSigninViewControllerDelegate
@@ -330,7 +330,7 @@ const CGFloat kFadeOutAnimationDuration = 0.16f;
   [self.unifiedConsentCoordinator resetSettingLinkTapped];
   self.unifiedConsentCoordinator.uiDisabled = NO;
   [self.viewController signinDidStop];
-  [self.viewController updatePrimaryButtonStyle];
+  [self.viewController setConfirmationButtonProperties];
 }
 
 #pragma mark - Private

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

@@ -48,8 +48,10 @@
 // of the screen.
 - (void)markUnifiedConsentScreenReachedBottom;
 
-// Updates the primary button based on the user sign-in state.
-- (void)updatePrimaryButtonStyle;
+// Sets the title, styling, and other button properties for the confirmation
+// button based on the user consent text that is currently displayed on-screen
+// and the whether the user has previously been signed-in.
+- (void)setConfirmationButtonProperties;
 
 // Returns the supported orientations for the device type:
 // |UIInterfaceOrientationPortrait| orientation on iPhone and all other