Fix app_list_demo compile and build in gn_all

I ran 'autoninja -c out/Default' and hit compile errors in app_list_demo.
AFAICT, app_list_demo is not built in gn_all (there's no path between them):
  gn path -C out/Default/ :gn_all //ui/app_list:app_list_demo
So, this target probably isn't built by the CQ and thus can break easily.

I fixed the errors and added the target to gn_all.

Bug: NONE
Test: 'autoninja -c out/Default' works; app_list_demo is built by bots
Change-Id: Ia1fb66e182a1dca2f1cd5abc236b20f98a3649a6
Reviewed-on: https://chromium-review.googlesource.com/902742
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534554}

diff --git a/BUILD.gn b/BUILD.gn
index ebb8c164..1961e2071 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -369,6 +369,7 @@
       "//chromeos:chromeos_unittests",
       "//chromeos/components:chromeos_components_unittests",
       "//components/session_manager/core",
+      "//ui/app_list:app_list_demo",
       "//ui/app_list:app_list_unittests",
       "//ui/app_list/presenter:app_list_presenter_unittests",
       "//ui/arc:ui_arc_unittests",