Move extension_messages.h to extensions/common.

extensions/browser/DEPS

 include_rules = [
   "+components/browser_context_keyed_service",
   "+components/user_prefs",
   "+content/public/browser",
   "+net",
   "+sync",
   "+third_party/leveldatabase",
 
   # Temporarily allowed includes as part of the app shell/extensions refactor.
   #
   # NOTE: Please do not add includes without talking to the app shell team;
   # see OWNERS for this directory.
   #
   # TODO(jamescook): Remove these. http://crbug.com/162530
   "+chrome/browser/chrome_notification_types.h",
   "+chrome/browser/extensions/api/content_settings/content_settings_store.h",
   "+chrome/browser/extensions/api/dns/host_resolver_wrapper.h",
   "+chrome/browser/extensions/api/preference/preference_api.h",
   "+chrome/browser/extensions/api/runtime/runtime_api.h",
   "+chrome/browser/extensions/extension_function_dispatcher.h",
   "+chrome/browser/extensions/extension_host.h",
   "+chrome/browser/extensions/extension_service.h",
   "+chrome/browser/renderer_host/chrome_render_message_filter.h",
   "+chrome/common/extensions/api/sockets/sockets_manifest_data.h",
-  "+chrome/common/extensions/extension_messages.h",
   "+chrome/common/extensions/features/feature_channel.h",
   "+chrome/common/extensions/permissions",
   "+grit/generated_resources.h",
 ]
 
 specific_include_rules = {
   "(.*test|.*test_util)\.(cc|h)": [
     # Temporarily allowed testing includes.  See above.
     # TODO(jamescook): Remove these. http://crbug.com/162530
     "+chrome/browser/extensions/extension_service_unittest.h",
     "+chrome/browser/extensions/test_extension_system.h",
     "+chrome/common/chrome_paths.h",
     "+chrome/test/base/testing_profile.h",
   ],
   # TODO(jamescook): Eliminate these after moving storage.idl to src/extensions.
   # http://crbug.com/348058
   "local_value_store_cache\.cc": [
     "+chrome/common/extensions/api/storage.h"
   ],
   "settings_frontend\.cc": [
     "+chrome/common/extensions/api/storage.h"
   ]
 }