Add Widevine signature files into Mac build

third_party/widevine/cdm/widevine.gni

 # Copyright 2015 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.
 
 declare_args() {
   # Allow widevinecdmadapter to be built in Chromium.
   enable_widevine = false
 }
+
+template("widevine_sign_file") {
+  # For official builds, generate a signature file in the same directory
+  # as |file| which will be used by Widevine.
+  action(target_name) {
+    forward_variables_from(invoker,
+                           [
+                             "file",
+                             "deps",
+                           ])
+    script = "//third_party/widevine/scripts/signature_generator.py"
+    sources = [
+      "$file",
+    ]
+    outputs = [
+      "$file.sig",
+    ]
+    args = [
+      "--input_file",
+      rebase_path("$file", root_build_dir),
+      "--output_file",
+      rebase_path("$file.sig", root_build_dir),
+    ]
+  }
+}