Preconditions:
1. Sign In to Chrome, with few accounts in SSO (A and B).

Steps to reproduce:
1. Cold start Chrome.
2. Go to Settings > Accounts > Add Account.
3. Add Account C to Chrome.
4. Navigate to https://accounts.google.com/SignOutOptions and observe A, B, and C are present.
5. Go to Settings > Accounts > Account C > Remove Account.
6. Navigate to https://accounts.google.com/SignOutOptions again and wait few seconds.

Observed results:
7. No accounts are available.

Expected results:
7. Accounts A and B are present, and only them. 

Additional comments:
GaiaCookieManagerService::ListAccounts returns its cached accounts unless they are marked as stale (via |list_accounts_stale_|).

Marking the listed accounts cache as stale is done:
* At the start of the GCMS.
* On cookie change notifications.

The later doesn't happen on iOS since Chrome is using WKWebView which doesn't have the necessary cookie APIs (we force it in very specific cases though).

The result is that adding an account or logging out all the accounts won't mark the listed accounts cache as stale, and GCMS::ListAccounts will keep returning the same accounts over and over.

The fix is to mark the cache as stale in GMCS::OnMergeSessionSuccess and GCMS::OnLogOutSuccess (the two requests that can modify the accounts available in the cookies). This won't have any effect on non-iOS platform, as the cache is already marked as stale when the cookie changes.