Add a GN flag for the Windows multi-dll build.

chrome/browser/policy/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.
 
-static_library("path_parser") {
+source_set("path_parser") {
   sources = [
     "policy_path_parser.h",
     "policy_path_parser_linux.cc",
     "policy_path_parser_mac.mm",
     "policy_path_parser_win.cc",
   ]
 
   deps = [
     "//base",
     "//chrome/common:constants",
     "//components/policy",
   ]
 }