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

content/renderer/render_view_impl.cc

Index: content/renderer/render_view_impl.cc
diff --git a/content/renderer/render_view_impl.cc b/content/renderer/render_view_impl.cc
index bbed24d2d818fbdbc90c23075963030bba771518..7546f74437a85122e10a785d3251be071ea274d8 100644
--- a/content/renderer/render_view_impl.cc
+++ b/content/renderer/render_view_impl.cc
@@ -3111,6 +3111,15 @@ void RenderViewImpl::willClose(WebFrame* frame) {
   FOR_EACH_OBSERVER(RenderViewObserver, observers_, FrameWillClose(frame));
 }
 
+void RenderViewImpl::didMatchCSS(
+    WebFrame* frame,
+    const WebVector<WebString>& newly_matching_selectors,
+    const WebVector<WebString>& stopped_matching_selectors) {
+  FOR_EACH_OBSERVER(
+      RenderViewObserver, observers_,
+      DidMatchCSS(frame, newly_matching_selectors, stopped_matching_selectors));
+}
+
 void RenderViewImpl::Repaint(const gfx::Size& size) {
   OnRepaint(size);
 }