[Signin iOS] Reload the Account Selector screen when identities change

The model was already being reloaded but not the the collection view,
this could cause crashes as the collection view and its model were not
in sync anymore. The collection view is now reloaded as well.

BUG=680098

Review-Url: https://codereview.chromium.org/2622103005
Cr-Commit-Position: refs/heads/master@{#443243}

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

@@ -256,6 +256,7 @@ typedef NS_ENUM(NSInteger, ItemType) {
 - (void)onIdentityListChanged {
   ChromeIdentity* selectedIdentity = [self selectedIdentity];
   [self loadModel];
+  [self.collectionView reloadData];
 
   // Reselect the identity.
   if (!selectedIdentity) {