Simplify isBrowserExtension() implementation

src/annotator/config/settings.js

@@ -111,13 +111,7 @@ function configFuncSettingsFrom(window_) {
  *
  */
 function isBrowserExtension(config) {
-  if (config.app.indexOf('chrome-extension://') === 0 ||
-    config.app.indexOf('moz-extension://') === 0 ||
-    config.app.indexOf('ms-browser-extension://') === 0) {
-    return true;
-  }
-
-  return false;
+  return !(config.app.startsWith('http://') || config.app.startsWith('https://'));
 }
 
 module.exports = {

src/annotator/config/test/settings-test.js

@@ -259,9 +259,10 @@ describe('annotation.config.settings', function() {
         url: 'https://partner.org',
         returns: false,
       },
+      // It considers anything not http(s) to be a browser extension.
       {
         url: 'ftp://partner.org',
-        returns: false,
+        returns: true,
       },
     ].forEach(function(test) {
       it('returns ' + test.returns + ' for ' + test.url, function() {