To avoid add so many lines when adding a new asset file, I'd suggest the xwalk_core_test_apk rewrite as following. 
```
    {
      'target_name': 'xwalk_core_test_apk',
      'type': 'none',
      'dependencies': [
        '../base/base.gyp:base_java_test_support',
        '../content/content.gyp:content_java_test_support',
        '../net/net.gyp:net_java_test_support',
        'xwalk_core_shell_apk_java',
        '../tools/android/md5sum/md5sum.gyp:md5sum',
        '../tools/android/forwarder2/forwarder.gyp:forwarder2',
      ],
      'variables': {
        'apk_name': 'XWalkCoreTest',
        'java_in_dir': 'test/android/core/javatests',
        'is_test_apk': 1,
        'additional_input_paths': [
          '<(PRODUCT_DIR)/xwalk_xwview_test/assets',
        ],
        'asset_location': '<(ant_build_out)/xwalk_xwview_test/assets',
      },
      'copies': [
        {
          'destination': '<(PRODUCT_DIR)/xwalk_xwview_test',
          'files': ['<(java_in_dir)/assets'],
        },
      ],
      'includes': [ '../build/java_apk.gypi' ],
    },
```