WebUI: Ensure that unpack_pak GN rule produces correct timestamps.

Bug: 762346
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5ea349b1e1c99ad0c1a2b9e0f63b5049b9c4f30f
Reviewed-on: https://chromium-review.googlesource.com/683458
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504295}

chrome/browser/resources/optimize_webui.gni

@@ -110,7 +110,7 @@ template("unpak") {
     ]
 
     outputs = [
-      "$target_gen_dir/${invoker.out_folder}",
+      "$target_gen_dir/${invoker.out_folder}/unpack.stamp",
     ]
 
     deps = invoker.deps

chrome/browser/resources/unpack_pak.py

@@ -11,6 +11,10 @@ import sys
 
 _HERE_PATH = os.path.join(os.path.dirname(__file__))
 
+# The name of a dummy file to be updated always after all other files have been
+# written. This file is declared as the "output" for GN's purposes
+_TIMESTAMP_FILENAME = os.path.join('unpack.stamp')
+
 
 _SRC_PATH = os.path.normpath(os.path.join(_HERE_PATH, '..', '..', '..'))
 sys.path.insert(1, os.path.join(_SRC_PATH, 'tools', 'grit'))
@@ -62,6 +66,10 @@ def main():
 
   unpack(args.pak_file, args.out_folder)
 
+  timestamp_file_path = os.path.join(args.out_folder, _TIMESTAMP_FILENAME)
+  with open(timestamp_file_path, 'a'):
+    os.utime(timestamp_file_path, None)
+
 
 if __name__ == '__main__':
   main()