Make BackgroundContentsService start up BackgroundContents with a delay, as for ExtensionHosts.

extensions/extensions.gypi

 # Copyright 2015 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.
 
 {
   'variables': {
     'extensions_common_constants_sources': [
       'common/constants.cc',
       'common/constants.h',
     ],
@@ skipping 522 matching lines @@
       'browser/content_hash_tree.h',
       'browser/content_verifier.cc',
       'browser/content_verifier.h',
       'browser/content_verifier_delegate.h',
       'browser/content_verifier_io_data.cc',
       'browser/content_verifier_io_data.h',
       'browser/content_verify_job.cc',
       'browser/content_verify_job.h',
       'browser/crx_file_info.cc',
       'browser/crx_file_info.h',
+      'browser/deferred_start_render_host.h',
       'browser/error_map.cc',
       'browser/error_map.h',
       'browser/event_listener_map.cc',
       'browser/event_listener_map.h',
       'browser/event_router.cc',
       'browser/event_router.h',
       'browser/extension_error.cc',
       'browser/extension_error.h',
       'browser/extension_function.cc',
       'browser/extension_function.h',
@@ skipping 452 matching lines @@
       'test/result_catcher.h',
       'test/test_content_utility_client.cc',
       'test/test_content_utility_client.h',
       'test/test_extensions_client.cc',
       'test/test_extensions_client.h',
       'test/test_permission_message_provider.cc',
       'test/test_permission_message_provider.h',
     ],
   },
 }