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

chrome/renderer/extensions/extension_helper.h

Index: chrome/renderer/extensions/extension_helper.h
diff --git a/chrome/renderer/extensions/extension_helper.h b/chrome/renderer/extensions/extension_helper.h
index 07a9b59f346c352aa1053d71adfdb5bc18e89043..c247d2f80211b5ba036da5bb2e0c45e06557acf4 100644
--- a/chrome/renderer/extensions/extension_helper.h
+++ b/chrome/renderer/extensions/extension_helper.h
@@ -63,6 +63,11 @@ class ExtensionHelper
   virtual void DidCreateDocumentElement(WebKit::WebFrame* frame) OVERRIDE;
   virtual void DidStartProvisionalLoad(WebKit::WebFrame* frame) OVERRIDE;
   virtual void FrameDetached(WebKit::WebFrame* frame) OVERRIDE;
+  virtual void DidMatchCSS(
+      WebKit::WebFrame* frame,
+      const WebKit::WebVector<WebKit::WebString>& newlyMatchingSelectors,
+      const WebKit::WebVector<WebKit::WebString>& stoppedMatchingSelectors)
+      OVERRIDE;
   virtual void DidCreateDataSource(WebKit::WebFrame* frame,
                                    WebKit::WebDataSource* ds) OVERRIDE;
   virtual void DraggableRegionsChanged(WebKit::WebFrame* frame) OVERRIDE;