Add LogOut method to GaiaCookieManagerService::Observer.

This allows GMCS observers to be notified whenever a logout operation
finished.

BUG=

Review-Url: https://codereview.chromium.org/2397263002
Cr-Commit-Position: refs/heads/master@{#423845}

components/signin/core/browser/gaia_cookie_manager_service.cc

@@ -666,8 +666,10 @@ void GaiaCookieManagerService::OnLogOutSuccess() {
   VLOG(1) << "GaiaCookieManagerService::OnLogOutSuccess";
 
   list_accounts_stale_ = true;
-
   fetcher_backoff_.InformOfRequest(true);
+  FOR_EACH_OBSERVER(Observer, observer_list_,
+                    OnLogOutAccountsFromCookieCompleted(
+                        GoogleServiceAuthError(GoogleServiceAuthError::NONE)));
   HandleNextRequest();
 }
 
@@ -687,6 +689,8 @@ void GaiaCookieManagerService::OnLogOutFailure(
     return;
   }
 
+  FOR_EACH_OBSERVER(Observer, observer_list_,
+                    OnLogOutAccountsFromCookieCompleted(error));
   HandleNextRequest();
 }
 

components/signin/core/browser/gaia_cookie_manager_service.h

@@ -80,11 +80,16 @@ class GaiaCookieManagerService : public KeyedService,
    public:
     // Called whenever a merge session is completed.  The account that was
     // merged is given by |account_id|.  If |error| is equal to
-    // GoogleServiceAuthError::AuthErrorNone() then the merge succeeeded.
+    // GoogleServiceAuthError::AuthErrorNone() then the merge succeeded.
     virtual void OnAddAccountToCookieCompleted(
         const std::string& account_id,
         const GoogleServiceAuthError& error) {}
 
+    // Called whenever a logout is completed. If |error| is equal to
+    // GoogleServiceAuthError::AuthErrorNone() then the logout succeeded.
+    virtual void OnLogOutAccountsFromCookieCompleted(
+        const GoogleServiceAuthError& error) {}
+
     // Called whenever the GaiaCookieManagerService's list of GAIA accounts is
     // updated. The GCMS monitors the APISID cookie and triggers a /ListAccounts
     // call on change. The GCMS will also call ListAccounts upon the first call