GN Windows component build fixes.

components/policy/proto/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("//third_party/protobuf/proto_library.gni")
 
-proto_library("proto") {
+# The proto files need to be a component to avoid duplicate symbols the way the
+# dependency structure is currently. Since proto_library generates a source
+# set, here we link the result into a component.
+component("proto") {
+  public_deps = [
+    ":proto_internal",
+  ]
+}
+
+proto_library("proto_internal") {
+  visibility = [ ":proto" ]
+
   sources = [
     "device_management_backend.proto",
     "policy_signing_key.proto",
   ]
   if (!is_android && !is_ios) {
     sources += [ "chrome_extension_policy.proto" ]
   }
   if (is_chromeos) {
     sources += [ "device_management_local.proto" ]
   }
 
   proto_out_dir = "policy/proto"
   cc_generator_options = "dllexport_decl=POLICY_PROTO_EXPORT:"
   cc_include = "components/policy/policy_proto_export.h"
   defines = [ "POLICY_PROTO_COMPILATION" ]
 }