bug 1543115: remote: emit "remote-listening" system notification on startup; r=remote-protocol-reviewers,maja_zf

When calling RemoteAgent.listen() across XPIDL the function is run
asynchronously.  In order to find out when the remote agent has started
listening we introduce a "remote-listening" system observer notification.

Differential Revision: https://phabricator.services.mozilla.com/D50285

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

remote/RemoteAgent.jsm

@@ -91,6 +91,11 @@ class RemoteAgentClass {
 
       this.server._start(port, host);
       dump(`DevTools listening on ${mainTarget.wsDebuggerURL}\n`);
+      Services.obs.notifyObservers(
+        null,
+        "remote-listening",
+        mainTarget.wsDebuggerURL
+      );
     } catch (e) {
       await this.close();
       throw new Error(`Unable to start remote agent: ${e.message}`, e);