make v8_use_snapshot configurable in gn

build/features.gypi

 # Copyright 2013 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 21 matching lines @@
     'v8_enable_disassembler%': 0,
 
     'v8_enable_gdbjit%': 0,
 
     'v8_object_print%': 0,
 
     'v8_enable_verify_heap%': 0,
 
     'v8_trace_maps%': 0,
 
+    # Enable the snapshot feature, for fast context creation.
+    # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
     'v8_use_snapshot%': 'true',
 
     'v8_enable_verify_predictable%': 0,
 
     # With post mortem support enabled, metadata is embedded into libv8 that
     # describes various parameters of the VM for use by debuggers. See
     # tools/gen-postmortem-metadata.py for details.
     'v8_postmortem_support%': 'false',
 
     # Interpreted regexp engine exists as platform-independent alternative
@@ skipping 82 matching lines @@
         },
         'conditions': [
           ['v8_enable_handle_zapping==1', {
             'defines': ['ENABLE_HANDLE_ZAPPING',],
           }],
         ],  # conditions
       },  # Release
     },  # configurations
   },  # target_defaults
 }