Introduce sources.gni file with a list of used .proto files

Bug: 181318121
Change-Id: I2c3f9d4ae6069a7c0cd6aba2112a8e44e5c1dd2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2722439
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Andrew Shulaev <ddrone@google.com>
Cr-Commit-Position: refs/heads/master@{#860762}

base/tracing/protos/BUILD.gn

@@ -5,20 +5,21 @@
 import("//third_party/perfetto/gn/proto_library.gni")
 import("//third_party/protobuf/proto_library.gni")
 import("//tools/grit/grit_rule.gni")
+import("sources.gni")
 
 proto_library("chrome_track_event") {
   proto_in_dir = "//"
   proto_out_dir = "base/tracing/protos"
   import_dirs = [ "//third_party/perfetto/" ]
-  sources = [ "chrome_track_event.proto" ]
+  sources = chrome_track_event_sources
   generate_cc = false
   generate_python = false
   generate_descriptor = "chrome_track_event.descriptor"
 }
 
 protozero_library("chrome_track_event_zero") {
   perfetto_root_path = "//third_party/perfetto/"
-  sources = [ "chrome_track_event.proto" ]
+  sources = chrome_track_event_sources
   import_dirs = [ "//third_party/perfetto/" ]
   generator_plugin_options = "wrapper_namespace=pbzero"
   omit_protozero_dep = true

base/tracing/protos/sources.gni

@@ -0,0 +1,11 @@
+# Copyright (c) 2020 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.
+
+# List of .proto files used when defining Chromium extensions for typed events.
+# New files should be added to this list.
+#
+# DO NOT EDIT THIS AT THE MOMENT. This file has to be supported in Perfetto
+# first in order to be useful.
+# TODO(b/181318121): Implement the support in Perfetto and remove the warning.
+chrome_track_event_sources = [ "chrome_track_event.proto" ]