move v8_use_external_startup_data to standalone.gypi

build/features.gypi

Index: build/features.gypi
diff --git a/build/features.gypi b/build/features.gypi
index c5b7c425b14aedf1630e57573cce3528c3ff96c2..81157f79ef91e53c7bcb47817608344cefe419e5 100644
--- a/build/features.gypi
+++ b/build/features.gypi
@@ -59,10 +59,6 @@
     # Enable compiler warnings when using V8_DEPRECATED apis.
     'v8_deprecation_warnings%': 0,
 
-    # Use external files for startup data blobs:
-    # the JS builtins sources and the start snapshot.
-    'v8_use_external_startup_data%': 0,
-
     # Set to 1 to enable DCHECKs in release builds.
     'dcheck_always_on%': 0,
   },
@@ -96,6 +92,8 @@
         'defines': ['V8_I18N_SUPPORT',],
       }],
       ['v8_use_external_startup_data==1', {
+        # This gyp variable is either set in standalone.gypi or externally,
+        # but should never be set in both in the same build.
         'defines': ['V8_USE_EXTERNAL_STARTUP_DATA',],
       }],
       ['dcheck_always_on!=0', {