Support WebSocket in WebRequest API.

chrome/test/data/extensions/api_test/webrequest/test_types.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.
 
 function getStyleURL() {
   // This is not a style sheet, but it must still be classified as "stylesheet"
   // because it is loaded as a style sheet.
   return getServerURL('empty.html?as-style');
 }
 
@@ skipping 688 matching lines @@
       [['onBeforeRequest', 'onBeforeSendHeaders', 'onSendHeaders',
         'onHeadersReceived', 'onResponseStarted', 'onCompleted']], {
         urls: ['<all_urls>'], types: ['csp_report']
       });
 
     var frame = document.createElement('iframe');
     frame.src =
       getServerURL('extensions/api_test/webrequest/csp/violation.html');
     document.body.appendChild(frame);
   },
+
+  // Note: The 'websocket' type is tested separately in 'test_websocket.js' and
+  // 'test_websocket_auth.js'.
 ]);