Bug 1299411 - s/on/once/ in NativeApp's sendMessage r=aswan

This communicates more clearly that we expect only one message/event.

MozReview-Commit-ID: 8jDYBVwqQtz

--HG--
extra : rebase_source : acae1570537b17027901253afc2344ffabd7a9ee

toolkit/components/extensions/NativeMessaging.jsm

@@ -430,8 +430,8 @@ this.NativeApp = class extends EventEmitter {
 
   sendMessage(msg) {
     let responsePromise = new Promise((resolve, reject) => {
-      this.on("message", (what, msg) => { resolve(msg); });
+      this.once("message", (what, msg) => { resolve(msg); });
-      this.on("disconnect", (what, err) => { reject(err); });
+      this.once("disconnect", (what, err) => { reject(err); });
     });
 
     let result = this.startupPromise.then(() => {