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

chrome/browser/extensions/extension_browsertests_misc.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/file_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/utf_string_conversions.h"
 #include "chrome/browser/extensions/autoupdate_interceptor.h"
 #include "chrome/browser/extensions/extension_apitest.h"
 #include "chrome/browser/extensions/extension_browsertest.h"
@@ skipping 11 matching lines @@
 #include "chrome/common/chrome_paths.h"
 #include "chrome/common/extensions/extension_action.h"
 #include "chrome/common/url_constants.h"
 #include "chrome/test/base/ui_test_utils.h"
 #include "content/browser/renderer_host/render_view_host.h"
 #include "content/browser/site_instance.h"
 #include "content/browser/tab_contents/tab_contents.h"
 #include "content/public/browser/notification_service.h"
 #include "net/base/net_util.h"
 #include "net/test/test_server.h"
+#include "webkit/glue/webpreferences.h"
 
 #if defined(TOOLKIT_VIEWS)
 #include "chrome/browser/ui/views/frame/browser_view.h"
 #endif
 
 const std::string kSubscribePage = "/subscribe.html";
 const std::string kFeedPage = "files/feeds/feed.html";
 const std::string kFeedPageMultiRel = "files/feeds/feed_multi_rel.html";
 const std::string kNoFeedPage = "files/feeds/no_feed.html";
 const std::string kValidFeed0 = "files/feeds/feed_script.xml";
@@ skipping 72 matching lines @@
 // pages. See http://crbug.com/64512.
 IN_PROC_BROWSER_TEST_F(ExtensionBrowserTest, WebKitPrefsBackgroundPage) {
   ASSERT_TRUE(LoadExtension(
       test_data_dir_.AppendASCII("good").AppendASCII("Extensions")
                     .AppendASCII("behllobkkfkfnphdnhnkndlbkcpglgmj")
                     .AppendASCII("1.0.0.0")));
 
   ExtensionProcessManager* manager =
         browser()->profile()->GetExtensionProcessManager();
   ExtensionHost* host = FindHostWithPath(manager, "/backgroundpage.html", 1);
-  WebPreferences prefs = host->GetWebkitPrefs();
+  WebPreferences prefs =
+      static_cast<RenderViewHostDelegate*>(host->host_contents())->
+          GetWebkitPrefs();
   ASSERT_FALSE(prefs.experimental_webgl_enabled);
   ASSERT_FALSE(prefs.accelerated_compositing_enabled);
   ASSERT_FALSE(prefs.accelerated_2d_canvas_enabled);
 }
 
 // Tests that we can load page actions in the Omnibox.
 IN_PROC_BROWSER_TEST_F(ExtensionBrowserTest, PageAction) {
   ASSERT_TRUE(test_server()->Start());
 
   // This page action will not show an icon, since it doesn't specify one but
@@ skipping 709 matching lines @@
 
   EXPECT_EQ(extension->GetResourceURL("options.html"),
             tab_strip->GetTabContentsAt(1)->tab_contents()->GetURL());
 }
 
 //==============================================================================
 // STOP! Please do not add any more random-ass tests here. Create new files for
 // your tests grouped by functionality. Also, you should strongly consider using
 // ExtensionAPITest if possible.
 //==============================================================================