chromoting: Add JS externs for qunit.

remoting/remoting_webapp_compile.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.
 #
 # File in charge of Closure compiling remoting's webapp.
 
 {
   'variables': {
     'success_stamp': '<(PRODUCT_DIR)/<(_target_name)_jscompile.stamp',
     'success_stamp_bt': '<(PRODUCT_DIR)/<(_target_name)_bt_jscompile.stamp',
     'success_stamp_ut': '<(PRODUCT_DIR)/<(_target_name)_ut_jscompile.stamp',
+    'externs': [
+      '<(DEPTH)/third_party/closure_compiler/externs/chrome_extensions.js',
+      '<@(remoting_webapp_js_externs)',
+    ],
     'compiler_flags': [
       '--strict',
       '--no-single-file',
       '--externs',
-      '<(DEPTH)/third_party/closure_compiler/externs/chrome_extensions.js',
+      '<(externs)',
     ],
   },
   'actions': [
     {
       'action_name': 'Verify remoting webapp',
       'inputs': [
         'remoting_webapp_compile.gypi',
         'remoting_webapp_files.gypi',
         '<@(remoting_webapp_crd_js_files)',
         '<@(remoting_webapp_js_proto_files)',
@@ skipping 47 matching lines @@
         '<@(compiler_flags)',
         '--success-stamp', '<(success_stamp_ut)',
         '<@(remoting_webapp_crd_js_files)',
         '<@(remoting_webapp_unittests_all_js_files)',
         '<@(remoting_webapp_unittests_js_proto_files)',
       ],
     },
   ],
   'includes': ['remoting_webapp_files.gypi'],
 }