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..69db5294cc8fa73b74f5f1ad3bb0a197ffcf512c 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("unpak") {
+  action(target_name) {
+    script = "//chrome/browser/resources/unpack_pak.py"
+
+    inputs = [
+      "//chrome/browser/resources/unpack_pak.py",
+    ]
+
+    outputs = [
+      "$target_gen_dir/${invoker.out_folder}",

[Nico, 2017/03/22 22:09:06]

Listing directories as outputs isn't great, since mtimes on directories are a
bit unpredictable (In particular, if a file in a subdirectory is touched, it
won't update the parent directories time stamp). Can you use a stamp file as
output instead?

+    ]
+
+    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),
+    ]
+  }
+}