Split ExtensionMessageFilter up into a UI thread part and an IO thread part.

extensions/browser/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 
 # GYP version: extensions/extensions.gyp:extensions_browser
 source_set("browser") {
   sources = []
@@ skipping 447 matching lines @@
       "image_loader.cc",
       "image_loader.h",
       "image_loader_factory.cc",
       "image_loader_factory.h",
       "info_map.cc",
       "info_map.h",
       "install/crx_installer_error.h",
       "install/extension_install_ui.cc",
       "install/extension_install_ui.h",
       "install_flag.h",
+      "io_thread_extension_message_filter.cc",
+      "io_thread_extension_message_filter.h",
       "lazy_background_task_queue.cc",
       "lazy_background_task_queue.h",
       "management_policy.cc",
       "management_policy.h",
       "mojo/keep_alive_impl.cc",
       "mojo/keep_alive_impl.h",
       "mojo/service_registration_manager.cc",
       "mojo/service_registration_manager.h",
       "mojo/stash_backend.cc",
       "mojo/stash_backend.h",
@@ skipping 134 matching lines @@
         "api/networking_private/networking_private_service_client.cc",
         "api/networking_private/networking_private_service_client.h",
       ]
     }
   }
 
   if (is_win) {
     cflags = [ "/wd4267" ]  # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   }
 }