login: use the same observer list between real and fake session_manager_client

In https://crrev.com/c/2459528 we added a test for container_client_adapter that uses fake session_manager_client therefore we need to ensure that fake works the same way as real.

BUG=b:164816080
TEST=components_unittests --gtest_filter='ArcContainerClientAdapterTest*'
TEST=revert the fix and ensure that the test fails

Change-Id: I271d116d10387cb6b5967378bab3fdea508d6581
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461603
Auto-Submit: Muhammad Hasan Khan <mhasank@chromium.org>
Commit-Queue: Muhammad Hasan Khan <mhasank@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816910}

chromeos/dbus/session_manager/fake_session_manager_client.h

@@ -288,7 +288,8 @@ class COMPONENT_EXPORT(SESSION_MANAGER) FakeSessionManagerClient
   // requested restarted arguments.
   base::Optional<std::vector<std::string>> restart_job_argv_;
 
-  base::ObserverList<Observer>::Unchecked observers_;
+  base::ObserverList<Observer>::Unchecked observers_{
+      SessionManagerClient::kObserverListPolicy};
   SessionManagerClient::ActiveSessionsMap user_sessions_;
   std::vector<std::string> server_backed_state_keys_;
 

chromeos/dbus/session_manager/session_manager_client.cc

@@ -1062,7 +1062,7 @@ class SessionManagerClientImpl : public SessionManagerClient {
   dbus::ObjectProxy* session_manager_proxy_ = nullptr;
   std::unique_ptr<BlockingMethodCaller> blocking_method_caller_;
   base::ObserverList<Observer>::Unchecked observers_{
-      base::ObserverListPolicy::EXISTING_ONLY};
+      SessionManagerClient::kObserverListPolicy};
 
   // Most recent screen-lock state received from session_manager.
   bool screen_is_locked_ = false;

chromeos/dbus/session_manager/session_manager_client.h

@@ -41,6 +41,9 @@ namespace chromeos {
 // SessionManagerClient is used to communicate with the session manager.
 class COMPONENT_EXPORT(SESSION_MANAGER) SessionManagerClient {
  public:
+  static constexpr base::ObserverListPolicy kObserverListPolicy =
+      base::ObserverListPolicy::EXISTING_ONLY;
+
   // The result type received from session manager on request to retrieve the
   // policy. Used to define the buckets for an enumerated UMA histogram.
   // Hence,