Simplify ContentWatcher by calling querySelector directly instead of through JS.

chrome/renderer/resources/extensions/content_watcher.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 contentWatcherNative = requireNative("contentWatcherNative");
 
-// Returns the indices in |cssSelectors| that match any element on the page.
-exports.FindMatchingSelectors = function(cssSelectors) {
-  var result = []
-  $Array.forEach(cssSelectors, function(selector, index) {
-    try {
-      if (document.querySelector(selector) != null)
-        $Array.push(result, index);
-    } catch (exception) {
-      throw new Error("query Selector failed on '" + selector + "': " +
-                      exception.stack);
-    }
-  });
-  return result;
-};
-
 // Watches the page for all changes and calls FrameMutated (a C++ callback) in
 // response.
 var mutation_observer = new MutationObserver(contentWatcherNative.FrameMutated);
 
 // This runs once per frame, when the module is 'require'd.
 mutation_observer.observe(document, {
   childList: true,
   attributes: true,
   characterData: true,
   subtree: true});