bug 1498758 - Update placeholder value of DNS over HTTPS input, add default pref value r=jduell

Update placeholder value of DNS over HTTPS input to the cloudflare provider add a default pref value for network.trr.uri

Differential Revision: https://phabricator.services.mozilla.com/D8639

browser/components/preferences/connection.js

@@ -42,6 +42,7 @@ window.addEventListener("DOMContentLoaded", () => {
     gConnectionsDialog.proxyTypeChanged.bind(gConnectionsDialog));
   Preferences.get("network.proxy.socks_version").on("change",
     gConnectionsDialog.updateDNSPref.bind(gConnectionsDialog));
+  gConnectionsDialog.initDnsOverHttpsUI();
 
   document
     .getElementById("disableProxyExtension")
@@ -310,4 +311,9 @@ var gConnectionsDialog = {
     // turn an empty string into `undefined` to clear the pref back to the default
     return uriString.length ? uriString : undefined;
   },
+
+  initDnsOverHttpsUI() {
+    let input = document.getElementById("networkDnsOverHttpsUrl");
+    input.placeholder = Preferences.get("network.trr.uri").defaultValue;
+  },
 };