Update references to the extension messaging APIs to point to the "runtime"

chrome/common/extensions/docs/examples/api/devtools/audits/broken-links/devtools.js

 // Copyright (c) 2012 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.
 
 var category = chrome.experimental.devtools.audits.addCategory(
     "Broken links", 1);
 category.onAuditStarted.addListener(function callback(auditResults) {
   chrome.extension.sendRequest({ tabId: webInspector.inspectedWindow.tabId },
       function(results) {
     if (!results.badlinks.length) {
       auditResults.addResult("No broken links",
                              "There are no broken links on the page!",
                              auditResults.Severity.Info);
     }
     else {
       var details = auditResults.createResult(results.badlinks.length +
           " links out of " + results.total + " are broken");
       for (var i = 0; i < results.badlinks.length; ++i) {
         details.addChild(auditResults.createURL(results.badlinks[i].href,
                                                 results.badlinks[i].text));
       }
       auditResults.addResult("Broken links found (" +
                                  results.badlinks.length +
                                  ")", "",
                              auditResults.Severity.Severe,
                              details);
     }
     auditResults.done();
   });
-});
+});