Have ExtensionHost use TabContents instead of RenderViewHost. Try #3.

content/browser/tab_contents/tab_contents.cc

Index: content/browser/tab_contents/tab_contents.cc
diff --git a/content/browser/tab_contents/tab_contents.cc b/content/browser/tab_contents/tab_contents.cc
index 48c23d7e9f05afd0291308dd9d6536b1911d3e73..03a6ed474bf377a1cb8a33d52d0426fddbe30515 100644
--- a/content/browser/tab_contents/tab_contents.cc
+++ b/content/browser/tab_contents/tab_contents.cc
@@ -203,7 +203,6 @@ TabContents::TabContents(content::BrowserContext* browser_context,
       temporary_zoom_settings_(false),
       content_restrictions_(0),
       view_type_(content::VIEW_TYPE_TAB_CONTENTS) {
-
   render_manager_.Init(browser_context, site_instance, routing_id);
 
   // We have the initial size of the view be based on the size of the passed in
@@ -1704,6 +1703,12 @@ void TabContents::DidChangeLoadProgress(double progress) {
     delegate()->LoadProgressChanged(progress);
 }
 
+void TabContents::DocumentAvailableInMainFrame(
+    RenderViewHost* render_view_host) {
+  FOR_EACH_OBSERVER(TabContentsObserver, observers_,
+                    DocumentAvailableInMainFrame());
+}
+
 void TabContents::DocumentOnLoadCompletedInMainFrame(
     RenderViewHost* render_view_host,
     int32 page_id) {