Support external startup data in V8.

build/features.gypi

Index: build/features.gypi
diff --git a/build/features.gypi b/build/features.gypi
index e8f5b2f08f336b0a7794186d724307a7b9a0130c..3b0af913fdf34f9ba6cbdc1efdb60b834d752bf6 100644
--- a/build/features.gypi
+++ b/build/features.gypi
@@ -59,6 +59,10 @@
 
     # Use the v8 provided v8::Platform implementation.
     'v8_use_default_platform%': 1,
+
+    # Use external files for startup data blobs:
+    # the JS builtins sources and the start snapshot.
+    'v8_use_external_startup_data%': 0,
   },
   'target_defaults': {
     'conditions': [
@@ -87,9 +91,10 @@
         'defines': ['V8_USE_DEFAULT_PLATFORM',],
       }],
       ['v8_compress_startup_data=="bz2"', {
-        'defines': [
-          'COMPRESS_STARTUP_DATA_BZ2',
-        ],
+        'defines': ['COMPRESS_STARTUP_DATA_BZ2',],
+      }],
+      ['v8_use_external_startup_data==1', {
+        'defines': ['V8_USE_EXTERNAL_STARTUP_DATA',],
       }],
     ],  # conditions
     'configurations': {