Use a depfile to know when to regenerate notice files

components/resources/BUILD.gn

 # Copyright 2014 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("//printing/features/features.gni")
 import("//tools/grit/grit_rule.gni")
 import("//third_party/brotli/brotli.gni")
 
 about_credits_file = "$target_gen_dir/about_credits.html"
 about_credits_file_bro = "$target_gen_dir/about_credits.bro"
@@ skipping 46 matching lines @@
 compress_file_brotli("compressed_about_credits") {
   input_file = about_credits_file
   output_file = about_credits_file_bro
   deps = [
     ":about_credits",
   ]
 }
 
 action("about_credits") {
   script = "//tools/licenses.py"
+  depfile = "$target_gen_dir/$target_name.d"
 
   inputs = [
-    # This is not a complete list. TODO(phajdan.jr, dbeam): licenses.py needs to
-    # generate a .d file with all the licenses/credits that about:credits uses.
-    # Then about:credits will automatically rebuild when one of them changes.
-    # See: depfile in gn's documentation (gn help depfile).
     "../about_ui/resources/about_credits.tmpl",
     "../about_ui/resources/about_credits_entry.tmpl",
   ]
 
   outputs = [
     about_credits_file,
   ]
 
   args = [
     "--target-os=$target_os",
+    "--depfile",
+    rebase_path(depfile, root_build_dir),
     "credits",
     rebase_path(about_credits_file, root_build_dir),
   ]
 }