Add ES2015 RegExp full subclassing semantics behind a flag

tools/gyp/v8.gyp

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 2063 matching lines @@
           '../../src/debug/mirrors.js',
           '../../src/debug/debug.js',
           '../../src/debug/liveedit.js',
         ],
         'experimental_library_files': [
           '../../src/js/macros.py',
           '../../src/messages.h',
           '../../src/js/generator.js',
           '../../src/js/harmony-atomics.js',
           '../../src/js/harmony-regexp.js',
+          '../../src/js/harmony-regexp-exec.js',
           '../../src/js/harmony-object-observe.js',
           '../../src/js/harmony-sharedarraybuffer.js',
           '../../src/js/harmony-simd.js',
           '../../src/js/harmony-species.js',
           '../../src/js/harmony-unicode-regexps.js',
           '../../src/js/promise-extra.js',
         ],
         'libraries_bin_file': '<(SHARED_INTERMEDIATE_DIR)/libraries.bin',
         'libraries_experimental_bin_file': '<(SHARED_INTERMEDIATE_DIR)/libraries-experimental.bin',
         'libraries_extras_bin_file': '<(SHARED_INTERMEDIATE_DIR)/libraries-extras.bin',
@@ skipping 183 matching lines @@
         }],
         ['want_separate_host_toolset==1', {
           'toolsets': ['host'],
         }, {
           'toolsets': ['target'],
         }],
       ],
     },
   ],
 }