bug 1594351: toolkit: add AppConstants.REMOTE_AGENT; r=remote-protocol-reviewers,maja_zf,mconley

The remote agent is a forthcoming new low-level debugging interface in
Gecko.  This constant will tell toolkit code that the feature is enabled.

Differential Revision: https://phabricator.services.mozilla.com/D52357

--HG--
extra : moz-landing-system : lando

toolkit/modules/AppConstants.jsm

@@ -371,4 +371,11 @@ this.AppConstants = Object.freeze({
 #else
     false,
 #endif
+
+  ENABLE_REMOTE_AGENT:
+#ifdef ENABLE_REMOTE_AGENT
+    true,
+#else
+    false,
+#endif
 });