Test for existence of browser.dns

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/780

browser.dns is not available before Firefox 60,
test for presence.

platform/firefox/vapi-webrequest.js

@@ -167,7 +167,8 @@
             if (
                 typeof list !== 'string' ||
                 list.length === 0 ||
-                list === 'unset'
+                list === 'unset' ||
+                browser.dns instanceof Object === false
             ) {
                 return null;
             }