Let v8's configs have an effect in 64-bit Windows Chromium builds.

build/toolchain.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 1269 matching lines @@
       'Debug': {
         'inherit_from': ['DebugBaseCommon'],
         'conditions': [
           ['v8_optimized_debug==0', {
             'inherit_from': ['DebugBase0'],
           }, {
             'inherit_from': ['DebugBase1'],
           }],
         ],
       },  # Debug
-      'Release': {
+      'ReleaseBase': {
+        'abstract': 1,
         'variables': {
           'v8_enable_slow_dchecks%': 0,
         },
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd" \
             or OS=="aix"', {
             'cflags!': [
               '-Os',
             ],
             'cflags': [
@@ skipping 59 matching lines @@
               },
             },
           }],  # OS=="win"
           ['v8_enable_slow_dchecks==1', {
             'defines': [
               'ENABLE_SLOW_DCHECKS',
             ],
           }],
         ],  # conditions
       },  # Release
+      'Release': {
+        'inherit_from': ['ReleaseBase'],
+      },  # Debug
+      'conditions': [
+        [ 'OS=="win"', {
+          # TODO(bradnelson): add a gyp mechanism to make this more graceful.
+          'Debug_x64': {
+            'inherit_from': ['DebugBaseCommon'],
+            'conditions': [
+              ['v8_optimized_debug==0', {
+                'inherit_from': ['DebugBase0'],
+              }, {
+                'inherit_from': ['DebugBase1'],
+              }],
+            ],
+          },
+          'Release_x64': {
+            'inherit_from': ['ReleaseBase'],
+          },
+        }],
+      ],
     },  # configurations
   },  # target_defaults
 }