Make sure ar_sample_app is actually built in the GN build.

R=garykac@chromium.org
BUG=471916

Review URL: https://codereview.chromium.org/1341993002

Cr-Commit-Position: refs/heads/master@{#348788}

remoting/BUILD.gn

@@ -20,9 +20,10 @@ group("remoting_all") {
   deps = [
     #"//remoting:remoting_browser_test_resources",
     "//remoting:remoting_unittests",
+
     #"//remoting:remoting_webapp_unittests",
     #"//app_remoting_test.gyp:ar_sample_test_driver",
-    #"//app_remoting_webapp.gyp:ar_sample_app",
+    "//remoting/webapp:ar_sample_app",
   ]
 
   # TODO(GYP): add is_mac