Add metrics for Webstore Gallery Widget app

components/chrome_apps/webstore_widget/app/compiled_resources.gyp

 # 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.
 {
   'targets': [
     {
       'target_name': 'main',
       'variables': {
         'depends': [
           '<@(cws_widget_container)',
           '../../../../third_party/jstemplate/compiled_resources.gyp:jstemplate',
           '../../../../ui/webui/resources/js/i18n_template_no_process.js',
           '../../../../ui/webui/resources/js/load_time_data.js',
         ],
         'externs': [
           '<(EXTERNS_DIR)/chrome_send.js',
           '<(EXTERNS_DIR)/chrome_extensions.js',
           '<(EXTERNS_DIR)/file_manager_private.js',
+          '<(EXTERNS_DIR)/metrics_private.js',
           '../externs/chrome_webstore_widget_private.js',
           '../externs/webview_tag.js'
         ]
       },
       'includes': [
         '../../../../third_party/closure_compiler/compile_js.gypi',
         '../cws_widget/compiled_resources.gypi'
       ]
     },
     {
       'target_name': 'background',
       'variables': {
         'externs': [
           '<(EXTERNS_DIR)/chrome_extensions.js',
           '../externs/chrome_webstore_widget_private.js'
         ]
       },
       'includes': [
         '../../../../third_party/closure_compiler/compile_js.gypi'
       ]
     }
   ]
 }