WebUI: Split unpack_pak functionality to its own GN action.

chrome/browser/resources/vulcanize.gni

Index: chrome/browser/resources/vulcanize.gni
diff --git a/chrome/browser/resources/vulcanize.gni b/chrome/browser/resources/vulcanize.gni
index f241c07d37983a613a61600f81911beebee84889..da02bac9dd3c5817adf3fb01d9386395a060f6a1 100644
--- a/chrome/browser/resources/vulcanize.gni
+++ b/chrome/browser/resources/vulcanize.gni
@@ -98,3 +98,26 @@ template("polymer_css_build") {
            ] + 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),
+    ]
+  }
+}