Revert of Remove wasm compile time option and enable wasm behind a runtime flag.

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 49 matching lines @@
 
     # Enable compiler warnings when using V8_DEPRECATED apis.
     'v8_deprecation_warnings%': 0,
 
     # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
     'v8_imminent_deprecation_warnings%': 0,
 
     # Set to 1 to enable DCHECKs in release builds.
     'dcheck_always_on%': 0,
 
+    # Set to 1 to enable building with wasm prototype.
+    'v8_wasm%': 0,
+
     # Enable/disable JavaScript API accessors.
     'v8_js_accessors%': 0,
   },
   'target_defaults': {
     'conditions': [
       ['v8_enable_disassembler==1', {
         'defines': ['ENABLE_DISASSEMBLER',],
       }],
       ['v8_enable_gdbjit==1', {
         'defines': ['ENABLE_GDB_JIT_INTERFACE',],
@@ skipping 21 matching lines @@
       }],
       ['v8_enable_i18n_support==1', {
         'defines': ['V8_I18N_SUPPORT',],
       }],
       ['v8_use_snapshot=="true" and v8_use_external_startup_data==1', {
         'defines': ['V8_USE_EXTERNAL_STARTUP_DATA',],
       }],
       ['dcheck_always_on!=0', {
         'defines': ['DEBUG',],
       }],
+      ['v8_wasm!=0', {
+        'defines': ['V8_WASM',],
+      }],
     ],  # conditions
     'configurations': {
       'DebugBaseCommon': {
         'abstract': 1,
         'variables': {
           'v8_enable_handle_zapping%': 1,
         },
         'conditions': [
           ['v8_enable_handle_zapping==1', {
             'defines': ['ENABLE_HANDLE_ZAPPING',],
           }],
         ],
       },  # Debug
       'Release': {
         'variables': {
           'v8_enable_handle_zapping%': 1,
         },
         'conditions': [
           ['v8_enable_handle_zapping==1', {
             'defines': ['ENABLE_HANDLE_ZAPPING',],
           }],
         ],  # conditions
       },  # Release
     },  # configurations
   },  # target_defaults
 }