Limit PDF helper extension to print preview only

chrome/browser/resources/pdf/browser_api.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 /**
  * Returns a promise that will resolve to the default zoom factor.
  * @param {!Object} streamInfo The stream object pointing to the data contained
  *     in the PDF.
@@ skipping 188 matching lines @@
     });
   }).then(function() { return BrowserApi.create(streamInfo, false); });
 }
 
 /**
  * Returns a promise that will resolve to a BrowserApi instance.
  * @return {Promise<BrowserApi>} A promise to a BrowserApi instance for the
  *     current environment.
  */
 function createBrowserApi() {
-  if (window.location.search)
+  if (location.search) {
+    if (location.ancestorOrigins[0] !== 'chrome://print') {
+      throw new Error('Refused to load the standalone extension.');
+    }
     return createBrowserApiForStandaloneExtension();
+  }
 
   return createBrowserApiForMimeHandlerView();
 }