Move v8_use_external_startup_data out of //build/config:feature_flags.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 48e61632633fffc8336937865792ea343193b366..049f640000feca3f2ffa83cb2b65cbb900a2089a 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -23,7 +23,12 @@ content_tests_gypi_values =
 # GYP version //content/content_tests.gypi:test_support_content
 source_set("test_support") {
   testonly = true
+
   configs += [ "//build/config:precompiled_headers" ]
+  if (v8_use_external_startup_data) {
+    configs += [ "//v8:external_startup_data" ]
+  }
+
   public_deps = [
     "//content/public/app:both",
     "//content/public/browser",
@@ -209,6 +214,10 @@ source_set("browsertest_support") {
   } else {
     deps += [ "//content/public/browser" ]
   }
+
+  if (v8_use_external_startup_data) {
+    configs += [ "//v8:external_startup_data" ]
+  }
 }
 
 mojom("content_test_mojo_bindings") {