Adjust the visibility of the standalone targets in the GN build.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 64d75020051038fb7ba03f3f6eed9de9340af3b3..a48e78d71d50ec3bf2c674bd075cdd52c7e58701 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -338,7 +338,12 @@ if (v8_use_external_startup_data) {
 }
 
 action("postmortem-metadata") {
-  visibility = [ ":*" ]  # Only targets in this file can depend on this.
+  # Only targets in this file and the top-level visibility target can
+  # depend on this.
+  visibility = [
+    ":*",
+    "//:gn_visibility",
+  ]
 
   script = "tools/gen-postmortem-metadata.py"
 
@@ -423,7 +428,12 @@ source_set("v8_nosnapshot") {
 }
 
 source_set("v8_snapshot") {
-  visibility = [ ":*" ]  # Only targets in this file can depend on this.
+  # Only targets in this file and the top-level visibility target can
+  # depend on this.
+  visibility = [
+    ":*",
+    "//:gn_visibility",
+  ]
 
   deps = [
     ":js2c",