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 6f4f90870915b5e2290a047811d99645bf84a447..910efef557bc0cf029887adc7a47d48b5a1ed778 100644
--- a/content/renderer/render_view_impl.cc
+++ b/content/renderer/render_view_impl.cc
@@ -3102,6 +3102,15 @@ void RenderViewImpl::willClose(WebFrame* frame) {
   FOR_EACH_OBSERVER(RenderViewObserver, observers_, FrameWillClose(frame));
 }
 
+void RenderViewImpl::didMatchCSS(
+    WebFrame* frame,
+    const WebVector<WebString>& newlyMatchingSelectors,
+    const WebVector<WebString>& stoppedMatchingSelectors) {
+  FOR_EACH_OBSERVER(
+      RenderViewObserver, observers_,
+      DidMatchCSS(frame, newlyMatchingSelectors, stoppedMatchingSelectors));
+}
+
 void RenderViewImpl::Repaint(const gfx::Size& size) {
   OnRepaint(size);
 }