Use new GN features in the build.

third_party/protobuf/proto_library.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Compile a protocol buffer.
 #
 # Protobuf parameters:
 #
 #   proto_out_dir (optional)
 #       Specifies the path suffix that output files are generated under. This
@@ skipping 126 matching lines @@
       "--cpp_out",
       "$cc_generator_options$rel_out_dir",
       "--python_out",
       rel_py_out_dir,
     ]
 
     deps = [
       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
     }
 
     public_configs = [ "//third_party/protobuf:using_proto" ]
 
     public_deps = [
       # The generated headers reference headers within protobuf_lite, so
       # dependencies must be able to find those headers too.
       "//third_party/protobuf:protobuf_lite",
     ]
     deps = [
       ":$action_name",
     ]
   }
 }