build: fix v8_enable_handle_zapping override

It was previously ignored by features.gypi and therefore
enabled by default for release builds.

See https://code.google.com/p/chromium/issues/detail?id=318206

PR-URL: https://github.com/nodejs/node/pull/2731
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

common.gypi

@@ -56,7 +56,7 @@
     'configurations': {
       'Debug': {
         'variables': {
-          'v8_enable_handle_zapping%': 1,
+          'v8_enable_handle_zapping': 1,
         },
         'defines': [ 'DEBUG', '_DEBUG' ],
         'cflags': [ '-g', '-O0' ],
@@ -83,7 +83,7 @@
       },
       'Release': {
         'variables': {
-          'v8_enable_handle_zapping%': 0,
+          'v8_enable_handle_zapping': 0,
         },
         'cflags': [ '-O3', '-ffunction-sections', '-fdata-sections' ],
         'conditions': [