Use new GN features in the build.

build/config/linux/pkg_config.gni

 # Copyright (c) 2013 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.
 
 import("//build/config/sysroot.gni")
 
 # Defines a config specifying the result of running pkg-config for the given
 # packages. Put the package names you want to query in the "packages" variable
 # inside the template invocation.
 #
@@ skipping 55 matching lines @@
     pkgresult = exec_script(pkg_config_script, args, "value")
     include_dirs = pkgresult[0]
     cflags = pkgresult[1]
 
     if (!defined(invoker.ignore_libs) || !invoker.ignore_libs) {
       libs = pkgresult[2]
       lib_dirs = pkgresult[3]
       ldflags = pkgresult[4]
     }
 
-    if (defined(invoker.defines)) {
-      defines = invoker.defines
-    }
-    if (defined(invoker.visibility)) {
-      visibility = invoker.visibility
-    }
+    forward_variables_from(invoker,
+                           [
+                             "defines",
+                             "visibility",
+                           ])
   }
 }