Ash Display: Prevent toggling privacy screen setting if it is policy managed.

BUG=1040648

Change-Id: I897158eb5098bd08d36b2a37b0a1731909ae51e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095669Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Gil Dekel <gildekel@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748431}

ash/display/privacy_screen_controller.cc

@@ -53,6 +53,14 @@ void PrivacyScreenController::SetEnabled(bool enabled) {
     return;
   }
 
+  // Do not set the policy if it is managed by policy. However, we still want to
+  // notify observers that a change was attempted in order to show a toast.
+  if (IsManaged()) {
+    for (Observer& observer : observers_)
+      observer.OnPrivacyScreenSettingChanged(GetEnabled());
+    return;
+  }
+
   if (active_user_pref_service_) {
     active_user_pref_service_->SetBoolean(prefs::kDisplayPrivacyScreenEnabled,
                                           enabled);