Fix typo in standalone.gypi

without the % we can't override the value which makes the Makefile thing
not work

R=vogelheim@chromium.org
LOG=n
BUG=none

Review URL: https://codereview.chromium.org/1147273002

Cr-Commit-Position: refs/heads/master@{#28553}

build/standalone.gypi

@@ -44,7 +44,7 @@
     'v8_enable_i18n_support%': 1,
     'v8_deprecation_warnings': 1,
     # TODO(jochen): Turn this on.
-    'v8_imminent_deprecation_warnings': 0,
+    'v8_imminent_deprecation_warnings%': 0,
     'msvs_multi_core_compile%': '1',
     'mac_deployment_target%': '10.5',
     'release_extra_cflags%': '',