Use Blink support to watch CSS selectors directly instead of using a MutationObserver.

chrome/renderer/resources/extensions/content_watcher.js

Index: chrome/renderer/resources/extensions/content_watcher.js
diff --git a/chrome/renderer/resources/extensions/content_watcher.js b/chrome/renderer/resources/extensions/content_watcher.js
deleted file mode 100644
index e34317710613ee7a2bd3c8124591e0247100577f..0000000000000000000000000000000000000000
--- a/chrome/renderer/resources/extensions/content_watcher.js
+++ /dev/null
@@ -1,16 +0,0 @@
-// 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");
-
-// 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});