bug 1600111: remote: make nsIRemoteAgent.listen() sync r=remote-protocol-reviewers,maja_zf

Interfaces exposed over XPIDL cannot be marked async, otherwise their
return values get lost.  This patch makes nsIRemoteAgent.listen()
synchronous by removing the use of async/await.

Unfortunately Rust does not yet support deserialising promises sent across
XPIDL, documented in https://bugzilla.mozilla.org/show_bug.cgi?id=1512319.

In an effort to retain the original API internally the function
now returns a promise.  This means the function can be awaited in
JS only, which we make use of in remote/test/browser/head.js.

Differential Revision: https://phabricator.services.mozilla.com/D55173

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

remote/RemoteAgent.jsm

@@ -31,7 +31,7 @@ class RemoteAgentClass {
     return !!this.server && !this.server.isStopped();
   }
 
-  async listen(url) {
+  listen(url) {
     if (!Preferences.get(ENABLED, false)) {
       throw new Error("Remote agent is disabled by preference");
     }
@@ -56,7 +56,7 @@ class RemoteAgentClass {
     }
 
     if (this.listening) {
-      return;
+      return Promise.resolve();
     }
 
     Preferences.set(RecommendedPreferences);
@@ -77,6 +77,10 @@ class RemoteAgentClass {
       delete this.server._handler._overridePaths[target.path];
     });
 
+    return this.asyncListen(host, port);
+  }
+
+  async asyncListen(host, port) {
     try {
       await this.targets.watchForTargets();