[iOS] Plugging user consent preferences to the Google service settings

When the switch is toggled, the preference is updated, but the cell
should not be reloaded (to let the animation finish).
The UI is reloaded only when a value update notification is received
without user action.

When the user consent is removed, all sections are expanded and all
cells inside are enabled to be modified.

https://drive.google.com/open?id=1hvi5LyGjBDODkRQymvC5sbUjSN9tafFz

Bug: 827072, 849838
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I5bc39e1ee80d8844e56900a7a22e8201a3e2d411
Reviewed-on: https://chromium-review.googlesource.com/1144934
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580830}

ios/chrome/browser/ui/settings/google_services_settings_consumer.h

@@ -16,6 +16,9 @@
 @property(nonatomic, strong, readonly)
     CollectionViewModel<CollectionViewItem*>* collectionViewModel;
 
+// Reloads |sections|.
+- (void)reloadSections:(NSIndexSet*)sections;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_SETTINGS_GOOGLE_SERVICES_SETTINGS_CONSUMER_H_

ios/chrome/browser/ui/settings/google_services_settings_mediator.h

@@ -29,7 +29,7 @@ class PrefService;
 
 // View controller.
 @property(nonatomic, weak) id<GoogleServicesSettingsConsumer> consumer;
-// Browser state.
+// Authentication service.
 @property(nonatomic, assign) AuthenticationService* authService;
 
 @end

ios/chrome/browser/ui/settings/google_services_settings_view_controller.mm

@@ -166,6 +166,10 @@ constexpr NSInteger kSectionOffset = 1000;
   return cell;
 }
 
+- (void)reloadSections:(NSIndexSet*)sections {
+  [self.collectionView reloadSections:sections];
+}
+
 #pragma mark - CollectionViewController
 
 - (void)loadModel {