Fix installing Chrome apps in offline Demo Mode

Use EXTERNAL_POLICY[_DOWNLOAD] for the manifest locations
so Chrome apps are allowed to install.

Bug: 925419
Change-Id: Ie65c76a28d8ebe5c63ee4d54cd2449b8759f9e18
Reviewed-on: https://chromium-review.googlesource.com/c/1437543
Auto-Submit: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Ben Wells <benwells@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626932}

chrome/browser/extensions/external_provider_impl.cc

@@ -740,8 +740,8 @@ void ExternalProviderImpl::CreateExternalProviders(
         base::MakeRefCounted<chromeos::DemoExtensionsExternalLoader>(cache_dir);
     std::unique_ptr<ExternalProviderImpl> demo_apps_provider =
         std::make_unique<ExternalProviderImpl>(
-            service, loader, profile, Manifest::EXTERNAL_PREF,
-            Manifest::EXTERNAL_PREF_DOWNLOAD, Extension::NO_FLAGS);
+            service, loader, profile, Manifest::EXTERNAL_POLICY,
+            Manifest::EXTERNAL_POLICY_DOWNLOAD, Extension::NO_FLAGS);
     demo_apps_provider->set_auto_acknowledge(true);
     demo_apps_provider->set_install_immediately(true);
     chromeos::DemoSession::Get()->SetExtensionsExternalLoader(loader);