[Android] Fix gn build for instrumentation tests.

tools/android/common/common.gyp

 # Copyright (c) 2012 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.
 
 {
+  'variables': {
+    'chromium_code': 1,
+  },
   'targets': [
     {
       'target_name': 'android_tools_common',
       'type': 'static_library',
       'toolsets': ['host', 'target'],
       'include_dirs': [
         '..',
         '../../..',
       ],
       'sources': [
         'adb_connection.cc',
         'adb_connection.h',
         'daemon.cc',
         'daemon.h',
         'net.cc',
         'net.h',
       ],
     },
   ],
 }