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

components/html_viewer/BUILD.gn

Index: components/html_viewer/BUILD.gn
diff --git a/components/html_viewer/BUILD.gn b/components/html_viewer/BUILD.gn
index b4d6f1d8c6a866db648783d0b3045fd2483290ba..13e88ecb9092c86e48b05852e793fe3c90391f9b 100644
--- a/components/html_viewer/BUILD.gn
+++ b/components/html_viewer/BUILD.gn
@@ -221,6 +221,10 @@ source_set("lib") {
     "//mojo/services/network:network",
     "//mojo/services/tracing",
   ]
+
+  if (v8_use_external_startup_data) {
+    configs += [ "//v8:external_startup_data" ]
+  }
 }
 
 mojo_native_application("html_viewer") {
@@ -333,6 +337,10 @@ test("html_viewer_unittests") {
     deps += [ ":build_unittest_assets" ]
     apk_asset_location = html_viewer_unittests_assets
   }
+
+  if (v8_use_external_startup_data) {
+    configs += [ "//v8:external_startup_data" ]
+  }
 }
 
 mojo_native_application("apptests") {