Only define V8_USE_EXTERNAL_SNAPSHOT_DATA if we actually use a snapshot

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

Review URL: https://codereview.chromium.org/1218153006

Cr-Commit-Position: refs/heads/master@{#29445}

build/features.gypi

@@ -97,7 +97,7 @@
       ['v8_enable_i18n_support==1', {
         'defines': ['V8_I18N_SUPPORT',],
       }],
-      ['v8_use_external_startup_data==1', {
+      ['v8_use_snapshot=="true" and v8_use_external_startup_data==1', {
         'defines': ['V8_USE_EXTERNAL_STARTUP_DATA',],
       }],
       ['dcheck_always_on!=0', {