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

chrome/browser/extensions/extension_host_mac.h

Index: chrome/browser/extensions/extension_host_mac.h
diff --git a/chrome/browser/extensions/extension_host_mac.h b/chrome/browser/extensions/extension_host_mac.h
index d688f9f4884bd67755eb82b2e48b7026ef4d986d..2425d824ff2e661e42c6d07edd6f4ae3f9b7800e 100644
--- a/chrome/browser/extensions/extension_host_mac.h
+++ b/chrome/browser/extensions/extension_host_mac.h
@@ -10,18 +10,14 @@
 
 class RenderWidgetHostView;
 
+// TODO(mpcomplete): I don't know what this does or if it is needed anymore,
+// now that ExtensionHost is restructured to rely on TabContents.
 class ExtensionHostMac : public ExtensionHost {
  public:
   ExtensionHostMac(const Extension* extension, SiteInstance* site_instance,
                    const GURL& url, content::ViewType host_type) :
       ExtensionHost(extension, site_instance, url, host_type) {}
   virtual ~ExtensionHostMac();
- protected:
-  virtual RenderWidgetHostView* CreateNewWidgetInternal(
-      int route_id,
-      WebKit::WebPopupType popup_type);
-  virtual void ShowCreatedWidgetInternal(RenderWidgetHostView* widget_host_view,
-                                         const gfx::Rect& initial_pos);
  private:
   virtual void UnhandledKeyboardEvent(const NativeWebKeyboardEvent& event);