chromeos: Fix SessionObserver::OnFirstSessionStarted() not firing

During OOBE and normal user login, when
SessionController::SetUserSessionOrder() is called there is an
existing user session, so last_active_account_id.is_valid() returns
true and the observer never fires.

I suspect this breaks some accessibility features in kiosk mode
(magnifier keyboard control) and for hotrod (long-press for
spoken feedback).

It's probably been broken since the observer was added in May:
https://chromium-review.googlesource.com/c/chromium/src/+/1070771

Bug: 884275
Test: added to ash_unittests
Change-Id: Idbe4f20b7426742703995a05c7ebb1c5c8f1a7c8
Reviewed-on: https://chromium-review.googlesource.com/1226876Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591450}

ash/session/session_controller.cc

@@ -341,9 +341,10 @@ void SessionController::SetUserSessionOrder(
 
   // Check active user change and notifies observers.
   if (user_sessions_[0]->session_id != active_session_id_) {
+    const bool is_first_session = active_session_id_ == 0u;
     active_session_id_ = user_sessions_[0]->session_id;
 
-    if (!last_active_account_id.is_valid()) {
+    if (is_first_session) {
       for (auto& observer : observers_)
         observer.OnFirstSessionStarted();
     }

ash/session/session_controller_unittest.cc

@@ -51,6 +51,8 @@ class TestSessionObserver : public SessionObserver {
     user_session_account_ids_.push_back(account_id);
   }
 
+  void OnFirstSessionStarted() override { first_session_started_ = true; }
+
   void OnSessionStateChanged(SessionState state) override { state_ = state; }
 
   void OnActiveUserPrefServiceChanged(PrefService* pref_service) override {
@@ -67,6 +69,7 @@ class TestSessionObserver : public SessionObserver {
 
   SessionState state() const { return state_; }
   const AccountId& active_account_id() const { return active_account_id_; }
+  bool first_session_started() const { return first_session_started_; }
   const std::vector<AccountId>& user_session_account_ids() const {
     return user_session_account_ids_;
   }
@@ -78,6 +81,7 @@ class TestSessionObserver : public SessionObserver {
  private:
   SessionState state_ = SessionState::UNKNOWN;
   AccountId active_account_id_;
+  bool first_session_started_ = false;
   std::vector<AccountId> user_session_account_ids_;
   PrefService* last_user_pref_service_ = nullptr;
 
@@ -172,6 +176,18 @@ TEST_F(SessionControllerTest, SimpleSessionInfo) {
   EXPECT_TRUE(controller()->IsRunningInAppMode());
 }
 
+TEST_F(SessionControllerTest, OnFirstSessionStarted) {
+  // Simulate chrome starting a user session.
+  mojom::SessionInfo info;
+  FillDefaultSessionInfo(&info);
+  SetSessionInfo(info);
+  UpdateSession(1u, "user1@test.com");
+  controller()->SetUserSessionOrder({1u});
+
+  // Observer is notified.
+  EXPECT_TRUE(observer()->first_session_started());
+}
+
 // Tests that the CanLockScreen is only true with an active user session.
 TEST_F(SessionControllerTest, CanLockScreen) {
   mojom::SessionInfo info;