Use new GN features in the build.

third_party/protobuf/proto_library.gni

Index: third_party/protobuf/proto_library.gni
diff --git a/third_party/protobuf/proto_library.gni b/third_party/protobuf/proto_library.gni
index dfa1df283e9ffe0c07ee7dbccfb7dc4f77ccd1e6..c9f100409084dfe40844cada9f708bb6fa39daf8 100644
--- a/third_party/protobuf/proto_library.gni
+++ b/third_party/protobuf/proto_library.gni
@@ -144,21 +144,18 @@ template("proto_library") {
       protoc_label,
     ]
 
-    if (defined(invoker.deps)) {
-      deps += invoker.deps
-    }
+    forward_variables_from(invoker, [ "deps" ])
   }
 
   source_set(target_name) {
-    if (defined(invoker.visibility)) {
-      visibility = invoker.visibility
-    }
+    forward_variables_from(invoker,
+                           [
+                             "visibility",
+                             "defines",
+                           ])
 
     sources = get_target_outputs(":$action_name")
 
-    if (defined(invoker.defines)) {
-      defines = invoker.defines
-    }
     if (defined(invoker.extra_configs)) {
       configs += invoker.extra_configs
     }