Move impl of identity.GetAccounts() extension API to its own file

chrome/browser/extensions/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/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
 import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//rlz/features/features.gni")
@@ skipping 220 matching lines @@
     "api/i18n/i18n_api.cc",
     "api/i18n/i18n_api.h",
     "api/identity/extension_token_key.cc",
     "api/identity/extension_token_key.h",
     "api/identity/gaia_web_auth_flow.cc",
     "api/identity/gaia_web_auth_flow.h",
     "api/identity/identity_api.cc",
     "api/identity/identity_api.h",
     "api/identity/identity_constants.cc",
     "api/identity/identity_constants.h",
+    "api/identity/identity_get_accounts_function.cc",
+    "api/identity/identity_get_accounts_function.h",
     "api/identity/identity_get_auth_token_function.cc",
     "api/identity/identity_get_auth_token_function.h",
     "api/identity/identity_get_profile_user_info_function.cc",
     "api/identity/identity_get_profile_user_info_function.h",
     "api/identity/identity_launch_web_auth_flow_function.cc",
     "api/identity/identity_launch_web_auth_flow_function.h",
     "api/identity/identity_mint_queue.cc",
     "api/identity/identity_mint_queue.h",
     "api/identity/identity_remove_cached_auth_token_function.cc",
     "api/identity/identity_remove_cached_auth_token_function.h",
@@ skipping 943 matching lines @@
       "api/braille_display_private/brlapi_connection.cc",
       "api/braille_display_private/brlapi_connection.h",
       "api/braille_display_private/brlapi_keycode_map.cc",
       "api/braille_display_private/brlapi_keycode_map.h",
     ]
     deps += [ "//build/linux/libbrlapi" ]
   } else {
     sources += [ "api/braille_display_private/braille_controller_stub.cc" ]
   }
 }