Default isEnabled to true.

At first, there is no value saved.  So it's undefined which is falsy.  Instead, start enabled.

Fixes #2562

src/bg/is-enabled.js

@@ -9,6 +9,7 @@ of Greasemonkey.
 let isEnabled = true;
 chrome.storage.local.get('globalEnabled', v => {
   isEnabled = v['globalEnabled'];
+  if ('undefined' == typeof isEnabled) isEnabled = true;
   setIcon();
 });