Add missing variable in gyp file.

src/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 14 matching lines @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 {
   'variables': {
     'v8_code': 1,
     'v8_random_seed%': 314159265,
     'v8_vector_stores%': 0,
     'embed_script%': "",
     'warmup_script%': "",
+    'v8_extra_library_files%': [],
     'v8_experimental_extra_library_files%': [],
     'v8_enable_inspector%': 0,
     'mksnapshot_exec': '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)mksnapshot<(EXECUTABLE_SUFFIX)',
     'mkpeephole_exec': '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)mkpeephole<(EXECUTABLE_SUFFIX)',
     'v8_os_page_size%': 0,
   },
   'includes': ['../gypfiles/toolchain.gypi', '../gypfiles/features.gypi', 'inspector/inspector.gypi'],
   'targets': [
     {
       'target_name': 'v8',
@@ skipping 2419 matching lines @@
       'conditions': [
         ['want_separate_host_toolset_mkpeephole==1', {
           'toolsets': ['host'],
         }, {
           'toolsets': ['target'],
         }],
       ],
     },
   ],
 }