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

chrome/browser/extensions/extension_browsertest.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 "chrome/browser/extensions/extension_browsertest.h"
 
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/file_path.h"
@@ skipping 26 matching lines @@
       extension_installs_observed_(0),
       target_page_action_count_(-1),
       target_visible_page_action_count_(-1) {
   EXPECT_TRUE(temp_dir_.CreateUniqueTempDir());
 }
 
 void ExtensionBrowserTest::SetUpCommandLine(CommandLine* command_line) {
   // This enables DOM automation for tab contentses.
   EnableDOMAutomation();
 
-  // This enables it for extension hosts.
-  ExtensionHost::EnableDOMAutomation();
-
   PathService::Get(chrome::DIR_TEST_DATA, &test_data_dir_);
   test_data_dir_ = test_data_dir_.AppendASCII("extensions");
 
 #if defined(OS_CHROMEOS)
   // This makes sure that we create the Default profile first, with no
   // ExtensionService and then the real profile with one, as we do when
   // running on chromeos.
   command_line->AppendSwitchASCII(switches::kLoginUser,
                                   "TestUser@gmail.com");
   command_line->AppendSwitchASCII(switches::kLoginProfile, "user");
@@ skipping 436 matching lines @@
         MessageLoopForUI::current()->Quit();
       }
       break;
     }
 
     default:
       NOTREACHED();
       break;
   }
 }