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

chrome/common/extensions/api/declarative_content.json

 // 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.
 
 [
   {
     "namespace": "declarativeContent",
     "description": "Use the <code>chrome.declarativeContent</code> API to take actions depending on the content of a page, without requiring permission to read the page's content.",
     "documentation_permissions_required": ["declarative", "declarativeContent"],
     "types": [
       {
         "id": "PageStateMatcher",
         "type": "object",
         "description": "Matches the state of a web page by various criteria.",
         "properties": {
           "pageUrl": {
             "$ref": "events.UrlFilter",
             "description": "Matches if the condition of the UrlFilter are fulfilled for the top-level URL of the page.",
             "optional": true
           },
           "css": {
             "type": "array",
             "optional": true,
-            "description": "Matches if all of the CSS selectors in the array match in a frame with the same origin as the page's main frame.  Note that listing hundreds of CSS selectors here can slow down web sites.",
+            "description": "Matches if all of the CSS selectors in the array match in a frame with the same origin as the page's main frame.  Note that listing hundreds of CSS selectors or CSS selectors that match hundreds of times per page can slow down web sites.",
             // TODO(jyasskin): Figure out if we want to require all
             // the selectors to match in the same frame, or allow several
             // frames to contribute to a match.
             "items": { "type": "string" }
-            // TODO(jyasskin): Validate that the selectors in this
-            // array are valid. Otherwise, we can get exceptions from
-            // content_watcher.js:FindMatchingSelectors() long after the
-            // rule is registered.
-//          },
+//        },
 //          TODO: "text": {
 //            "type": "array",
 //            "optional": true,
 //            "description": "Matches if all of the regular expressions in the array match text in the page. The regular expressions use the <a href=\"http://code.google.com/p/re2/wiki/Syntax\">RE2 syntax</a>.",
 //            "items": { "type": "string" }
           },
           "instanceType": {
             "type": "string", "enum": ["declarativeContent.PageStateMatcher"],
             "nodoc": true
           }
@@ skipping 21 matching lines @@
           "supportsRules": true,
           "conditions": ["declarativeContent.PageStateMatcher"],
           "actions": [
             "declarativeContent.ShowPageAction"
           ]
         }
       }
     ]
   }
 ]