Enable external V8 snapshot on Android through GN.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index c51709c24ce97419a6be64241c616917c635e477..602102aa0d6af3d575e9b9d892273bb6972ea4c3 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -6,6 +6,7 @@ import("//build/config/allocator.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
+import("//build/module_args/v8.gni")
 
 declare_args() {
   # When set, turns off the (normally-on) iterator debugging and related stuff
@@ -204,6 +205,9 @@ config("feature_flags") {
   if (proprietary_codecs) {
     defines += [ "USE_PROPRIETARY_CODECS" ]
   }
+  if (v8_use_external_startup_data) {
+    defines += [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
+  }
 }
 
 # Debug/release ----------------------------------------------------------------