Add protobuf serialization to DisplayItemList

cc/proto/BUILD.gn

 # 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.
 
 import("//third_party/protobuf/proto_library.gni")
 
 component("cc_proto") {
   public_deps = [
     ":proto_internal",
   ]
 }
 
 proto_library("proto_internal") {
   visibility = [ ":cc_proto" ]
 
   sources = [
     # TODO(dtrainor): Move protos to their correct packages once it's possible
     # to include protos from other directories/targets.
+    "display_item.proto",
     "point.proto",
     "pointf.proto",
     "rect.proto",
     "rectf.proto",
     "size.proto",
     "sizef.proto",
     "skregion.proto",
     "skrrect.proto",
     "skxfermode.proto",
     "transform.proto",
   ]
 
   cc_generator_options = "dllexport_decl=CC_PROTO_EXPORT:"
   cc_include = "cc/proto/cc_proto_export.h"
 
   defines = [ "CC_PROTO_IMPLEMENTATION" ]
 
   extra_configs = [ "//build/config/compiler:wexit_time_destructors" ]
 }