chromeos: update current_account_id_ when user switch happens

This got lost in the refactoring. Added test coverage.

BUG=756085,875111
TEST=covered by test

Change-Id: I1b9bbc78b2657a1378b55542786b88f8911a4b9e
Reviewed-on: https://chromium-review.googlesource.com/c/1351144Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611899}

ash/multi_user/multi_user_window_manager.cc

@@ -258,6 +258,9 @@ void MultiUserWindowManager::OnActiveUserSessionChanged(
   // This needs to be set before the animation starts.
   current_account_id_ = account_id;
 
+  if (delegate_)
+    delegate_->OnWillSwitchActiveAccount(current_account_id_);
+
   // Here to avoid a very nasty race condition, we must destruct any previously
   // created animation before creating a new one. Otherwise, the newly
   // constructed will hide all windows of the old user in the first step of the

chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos_unittest.cc

@@ -1589,6 +1589,20 @@ TEST_F(MultiUserWindowManagerChromeOSTest, WindowBoundsAfterTabletMode) {
   EXPECT_EQ(bounds, window(1)->bounds());
 }
 
+TEST_F(MultiUserWindowManagerChromeOSTest, AccountIdChangesAfterSwitch) {
+  SetUpForThisManyWindows(1);
+
+  const AccountId account1(AccountId::FromUserEmail("A"));
+  const AccountId account2(AccountId::FromUserEmail("B"));
+  AddTestUser(account1);
+  AddTestUser(account2);
+  SwitchActiveUser(account1);
+  EXPECT_EQ(account1, multi_user_window_manager()->GetCurrentUserForTest());
+
+  SwitchActiveUser(account2);
+  EXPECT_EQ(account2, multi_user_window_manager()->GetCurrentUserForTest());
+}
+
 class MultiUserWindowManagerChromeOSMashTest
     : public MultiUserWindowManagerChromeOSTest {
  public: