Bug 1540191 - Add ability to disable BITS download for an update using the advertised update.xml. r=bytesized

Differential Revision: https://phabricator.services.mozilla.com/D28156

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

toolkit/mozapps/update/UpdateService.jsm

@@ -3927,6 +3927,10 @@ Downloader.prototype = {
     this.isCompleteUpdate = this._patch.type == "complete";
 
     let canUseBits = this._canUseBits(this._patch);
+    // Allow the advertised update to disable BITS.
+    if (this._update.getProperty("disableBITS") != null) {
+      canUseBits = false;
+    }
     if (!canUseBits) {
       this._downloaderName = "internal";
     }