Have ExtensionHost use TabContents instead of RenderViewHost. Try #2. The first

content/browser/tab_contents/tab_contents_observer.cc

Index: content/browser/tab_contents/tab_contents_observer.cc
diff --git a/content/browser/tab_contents/tab_contents_observer.cc b/content/browser/tab_contents/tab_contents_observer.cc
index c18b1f381ea2ec3a6b641aa3720c8e0ed4d0eb3a..ac41d29819a77f3a4a05c8e8804630edcc9d0cbe 100644
--- a/content/browser/tab_contents/tab_contents_observer.cc
+++ b/content/browser/tab_contents/tab_contents_observer.cc
@@ -57,6 +57,9 @@ void TabContentsObserver::DidFailProvisionalLoad(
     const string16& error_description) {
 }
 
+void TabContentsObserver::DocumentAvailableInMainFrame() {
+}
+
 void TabContentsObserver::DocumentLoadedInFrame(int64 frame_id) {
 }