Add LogOut method to GaiaCookieManagerService::Observer.

components/signin/core/browser/gaia_cookie_manager_service.h

Index: components/signin/core/browser/gaia_cookie_manager_service.h
diff --git a/components/signin/core/browser/gaia_cookie_manager_service.h b/components/signin/core/browser/gaia_cookie_manager_service.h
index 47c673fc8b1ae9c4e19da1e634c062c6010a92fb..72f178d72bca8cd51a9ec497eeb6ef04856d4ccf 100644
--- a/components/signin/core/browser/gaia_cookie_manager_service.h
+++ b/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