Bug 1240615 - Make PushService protocol init method return a Promise. r=kitcambridge

I'd like to make `PushService.init` fallible, but it's not obvious how
to act on failure.  This patch at least allows each service to block
_startService until service-specific work initialization is complete.

dom/push/PushService.jsm

@@ -513,7 +513,7 @@ this.PushService = {
     console.debug("startService()");
 
     if (this._state != PUSH_SERVICE_ACTIVATING) {
-      return;
+      return Promise.reject();
     }
 
     this._service = service;
@@ -523,9 +523,11 @@ this.PushService = {
       this._db = this._service.newPushDB();
     }
 
-    this._service.init(options, this, serverURI);
-    this._startObservers();
-    return this._dropExpiredRegistrations();
+    return this._service.init(options, this, serverURI)
+      .then(() => {
+        this._startObservers();
+        return this._dropExpiredRegistrations();
+      });
   },
 
   /**

dom/push/PushServiceHttp2.jsm

@@ -572,6 +572,8 @@ this.PushServiceHttp2 = {
     console.debug("init()");
     this._mainPushService = aMainPushService;
     this._serverURI = aServerURL;
+
+    return Promise.resolve();
   },
 
   _retryAfterBackoff: function(aSubscriptionUri, retryAfter) {

dom/push/PushServiceWebSocket.jsm

@@ -299,6 +299,8 @@ this.PushServiceWebSocket = {
     this._requestTimeout = prefs.get("requestTimeout");
     this._adaptiveEnabled = prefs.get('adaptive.enabled');
     this._upperLimit = prefs.get('adaptive.upperLimit');
+
+    return Promise.resolve();
   },
 
   _reconnect: function () {