Don't use -O3 with sanitizers.

build/standalone.gypi

 # 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 17 matching lines @@
 # Definitions to be used when building stand-alone V8 binaries.
 
 {
   # We need to include toolchain.gypi here for third-party sources that don't
   # directly include it themselves.
   'includes': ['toolchain.gypi'],
   'variables': {
     'component%': 'static_library',
     'make_clang_dir%': '../third_party/llvm-build/Release+Asserts',
     'clang_xcode%': 0,
-    'asan%': 0,
-    'tsan%': 0,
     'visibility%': 'hidden',
     'v8_enable_backtrace%': 0,
     'v8_enable_i18n_support%': 1,
     'v8_deprecation_warnings': 1,
     'msvs_multi_core_compile%': '1',
     'mac_deployment_target%': '10.5',
     'variables': {
       'variables': {
         'variables': {
           'conditions': [
@@ skipping 422 matching lines @@
       ],
     }],
     ['clang==1 and OS=="win"', {
       'make_global_settings': [
         # On Windows, gyp's ninja generator only looks at CC.
         ['CC', '<(make_clang_dir)/bin/clang-cl'],
       ],
     }],
   ],
 }