WebUI: Split unpack_pak functionality to its own GN action.

chrome/browser/resources/vulcanize.gni

 # Copyright 2017 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.
 
 template("node") {
   action(target_name) {
     forward_variables_from(invoker, "*")
 
     # Declare dependencies to all involved tools.
     inputs += [
@@ skipping 80 matching lines @@
 
     deps = invoker.deps
 
     args = [
              "--out_folder",
              rebase_path(target_gen_dir, root_build_dir),
              "--input_files",
            ] + invoker.input_files + [ "--output_files" ] + invoker.output_files
   }
 }
+
+template("unpack_pak") {
+  action(target_name) {
+    script = "//chrome/browser/resources/unpack_pak.py"
+
+    inputs = [
+      "//chrome/browser/resources/unpack_pak.py",
+    ]
+
+    outputs = [
+      "$target_gen_dir/${invoker.out_folder}",
+    ]
+
+    deps = invoker.deps
+
+    args = [
+      "--out_folder",
+      rebase_path("$target_gen_dir/${invoker.out_folder}", root_build_dir),
+      "--pak_file",
+      rebase_path("$target_gen_dir/${invoker.pak_file}", root_build_dir),
+    ]
+  }
+}