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

chrome/browser/extensions/extension_browsertests_misc.cc

Index: chrome/browser/extensions/extension_browsertests_misc.cc
diff --git a/chrome/browser/extensions/extension_browsertests_misc.cc b/chrome/browser/extensions/extension_browsertests_misc.cc
index c6ef3a0982b3cf0b8707d3b69428a66edf6a27c9..0981108dbf83d237ed8a81325e37d7384d784479 100644
--- a/chrome/browser/extensions/extension_browsertests_misc.cc
+++ b/chrome/browser/extensions/extension_browsertests_misc.cc
@@ -29,6 +29,7 @@
 #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"
@@ -121,7 +122,9 @@ IN_PROC_BROWSER_TEST_F(ExtensionBrowserTest, WebKitPrefsBackgroundPage) {
   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);