Tracing v2: wire up stubs generated by the protozero protoc plugin

components/tracing/proto/BUILD.gn

 # Copyright 2016 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")
 
+tracing_proto_sources = [
+  "event.proto",
+  "events_chunk.proto",
+]
+
 # Stubs generated using custom plugin.
 proto_library("protos") {
   visibility = [ "//components/tracing/*" ]
 
   proto_in_dir = "."
-  sources = [
-    "event.proto",
-    "events_chunk.proto",
-  ]
+  sources = tracing_proto_sources
 
   generator_plugin_label =
       "//components/tracing/tools/proto_zero_plugin:proto_zero_plugin"
   generator_plugin_suffix = ".pbzero"
   generator_plugin_options = "wrapper_namespace=pbzero"
   generate_cc = false
   generate_python = false
 }
 
 # Official protobuf C++ stubs to test conformance.
 proto_library("golden_protos_for_tests") {
   visibility = [ "//components/tracing/*" ]
   testonly = true
 
   proto_in_dir = "."
   proto_out_dir = "components/tracing/test/golden_protos"
-  sources = [
-    "events_chunk.proto",
-  ]
+  sources = tracing_proto_sources
 
   generate_python = false
 }