Add proto definition for METADATA.chromium files.

METADATA.chromium are new files defined by a proto schema that
accommodates team, OS, and component information, and that is extensible
and structured, to replace the metadata information currently stored as
OWNERS files comments.

See the proposal for more details:
https://docs.google.com/document/d/17WMlceIMwge2ZiCvBWaBuk0w60YgieBd-ly3I8XsbzU/preview

Change-Id: I271b8f4cee03951c1fe3c64dad5aa0661234636a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248340
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779868}

tools/directory_metadata/README.md

+# Chromium METADATA files
+
+METADATA.chromium files are a source-focused mechanism by which owners can
+provide users of their code important information, including:
+
+* The team responsible for the code.
+* The Monorail component where bugs should be filed.
+* The OS type.
+
+METADATA.chromium files are structured protobuf files that are amenable to
+programmatic interaction.
+
+## Contents
+
+This directory contains the
+[proto definition](https://source.chromium.org/chromium/chromium/src/+/master:tools/directory_metadata/directory_metadata.proto)
+for METADATA.chromium files, which is the source of truth about
+METADATA.chromium file contents.
+
+Historical information can be found in the
+[original proposal](https://docs.google.com/document/d/17WMlceIMwge2ZiCvBWaBuk0w60YgieBd-ly3I8XsbzU/preview).
+
+## Usage
+
+METADATA.chromium files apply to all contents of a directory including its
+subdirectories.
+
+There is no inheritance mechanism, so any information in METADATA.chromium files
+in parent directories is ignored.
+
+For example, given the files below, the value of the `os` field for
+a/b/METADATA.chromium would be `OS_UNSPECIFIED` regardless of the contents of
+a/METADATA.chromium.
+
+**a/METADATA.chromium**
+```
+monorail {
+  project: "chromium"
+  component: "Component"
+}
+team_email: "team@chromium.org"
+os: OS_LINUX
+```
+
+**a/b/METADATA.chromium**
+```
+monorail {
+  project: "chromium"
+  component: "Component>Foo"
+}
+team_email: "foo-team@chromium.org"
+```

tools/directory_metadata/directory_metadata.proto

+// Copyright 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.
+
+syntax = "proto3";
+
+// Metadata information for a directory.
+message Metadata {
+  // Monorail metadata. Its component field should always map to the same
+  // team_email field (see below) for every directory that it is listed in.
+  Monorail monorail = 1;
+  // The team responsible for the directory. Only a single email is allowed.
+  // For example, “team-email@chromium.org”.
+  string team_email = 2;
+  // The code in this directory is specific to this OS.
+  OS os = 3;
+  // Reserved for metadata information for third-party code. See
+  // https://opensource.google/docs/thirdparty/metadata/
+  reserved 13;
+  reserved "third_party";
+}
+
+// Supported OS to tag a directory.
+enum OS {
+  OS_UNSPECIFIED = 0;
+  OS_LINUX = 1;
+  OS_WINDOWS = 2;
+  OS_MAC = 3;
+  OS_ANDROID = 4;
+  OS_IOS = 5;
+  OS_CHROME = 6;
+  OS_FUCHSIA = 7;
+}
+
+// Monorail specific metadata.
+message Monorail {
+  // The Monorail project. For example, “chromium”.
+  string project = 1;
+  // The Monorail component. For example, “UI>Browser>Zoom”.
+  // Only a single component is allowed.
+  string component = 2;
+}

tools/directory_metadata/directory_metadata_pb2.py

+# Generated by the protocol buffer compiler.  DO NOT EDIT!
+# source: directory_metadata.proto
+
+import sys
+_b=sys.version_info[0]<3 and (lambda x:x) or (lambda x:x.encode('latin1'))
+from google.protobuf.internal import enum_type_wrapper
+from google.protobuf import descriptor as _descriptor
+from google.protobuf import message as _message
+from google.protobuf import reflection as _reflection
+from google.protobuf import symbol_database as _symbol_database
+# @@protoc_insertion_point(imports)
+
+_sym_db = _symbol_database.Default()
+
+
+
+
+DESCRIPTOR = _descriptor.FileDescriptor(
+  name='directory_metadata.proto',
+  package='',
+  syntax='proto3',
+  serialized_options=None,
+  serialized_pb=_b('\n\x18\x64irectory_metadata.proto\"_\n\x08Metadata\x12\x1b\n\x08monorail\x18\x01 \x01(\x0b\x32\t.Monorail\x12\x12\n\nteam_email\x18\x02 \x01(\t\x12\x0f\n\x02os\x18\x03 \x01(\x0e\x32\x03.OSJ\x04\x08\r\x10\x0eR\x0bthird_party\".\n\x08Monorail\x12\x0f\n\x07project\x18\x01 \x01(\t\x12\x11\n\tcomponent\x18\x02 \x01(\t*h\n\x02OS\x12\x12\n\x0eOS_UNSPECIFIED\x10\x00\x12\t\n\x05LINUX\x10\x01\x12\x0b\n\x07WINDOWS\x10\x02\x12\x07\n\x03MAC\x10\x03\x12\x0b\n\x07\x41NDROID\x10\x04\x12\x07\n\x03IOS\x10\x05\x12\n\n\x06\x43HROME\x10\x06\x12\x0b\n\x07\x46UCHSIA\x10\x07\x62\x06proto3')
+)
+
+_OS = _descriptor.EnumDescriptor(
+  name='OS',
+  full_name='OS',
+  filename=None,
+  file=DESCRIPTOR,
+  values=[
+    _descriptor.EnumValueDescriptor(
+      name='OS_UNSPECIFIED', index=0, number=0,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='LINUX', index=1, number=1,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='WINDOWS', index=2, number=2,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='MAC', index=3, number=3,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='ANDROID', index=4, number=4,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='IOS', index=5, number=5,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='CHROME', index=6, number=6,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='FUCHSIA', index=7, number=7,
+      serialized_options=None,
+      type=None),
+  ],
+  containing_type=None,
+  serialized_options=None,
+  serialized_start=173,
+  serialized_end=277,
+)
+_sym_db.RegisterEnumDescriptor(_OS)
+
+OS = enum_type_wrapper.EnumTypeWrapper(_OS)
+OS_UNSPECIFIED = 0
+LINUX = 1
+WINDOWS = 2
+MAC = 3
+ANDROID = 4
+IOS = 5
+CHROME = 6
+FUCHSIA = 7
+
+
+
+_METADATA = _descriptor.Descriptor(
+  name='Metadata',
+  full_name='Metadata',
+  filename=None,
+  file=DESCRIPTOR,
+  containing_type=None,
+  fields=[
+    _descriptor.FieldDescriptor(
+      name='monorail', full_name='Metadata.monorail', index=0,
+      number=1, type=11, cpp_type=10, label=1,
+      has_default_value=False, default_value=None,
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
+    _descriptor.FieldDescriptor(
+      name='team_email', full_name='Metadata.team_email', index=1,
+      number=2, type=9, cpp_type=9, label=1,
+      has_default_value=False, default_value=_b("").decode('utf-8'),
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
+    _descriptor.FieldDescriptor(
+      name='os', full_name='Metadata.os', index=2,
+      number=3, type=14, cpp_type=8, label=1,
+      has_default_value=False, default_value=0,
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
+  ],
+  extensions=[
+  ],
+  nested_types=[],
+  enum_types=[
+  ],
+  serialized_options=None,
+  is_extendable=False,
+  syntax='proto3',
+  extension_ranges=[],
+  oneofs=[
+  ],
+  serialized_start=28,
+  serialized_end=123,
+)
+
+
+_MONORAIL = _descriptor.Descriptor(
+  name='Monorail',
+  full_name='Monorail',
+  filename=None,
+  file=DESCRIPTOR,
+  containing_type=None,
+  fields=[
+    _descriptor.FieldDescriptor(
+      name='project', full_name='Monorail.project', index=0,
+      number=1, type=9, cpp_type=9, label=1,
+      has_default_value=False, default_value=_b("").decode('utf-8'),
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
+    _descriptor.FieldDescriptor(
+      name='component', full_name='Monorail.component', index=1,
+      number=2, type=9, cpp_type=9, label=1,
+      has_default_value=False, default_value=_b("").decode('utf-8'),
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
+  ],
+  extensions=[
+  ],
+  nested_types=[],
+  enum_types=[
+  ],
+  serialized_options=None,
+  is_extendable=False,
+  syntax='proto3',
+  extension_ranges=[],
+  oneofs=[
+  ],
+  serialized_start=125,
+  serialized_end=171,
+)
+
+_METADATA.fields_by_name['monorail'].message_type = _MONORAIL
+_METADATA.fields_by_name['os'].enum_type = _OS
+DESCRIPTOR.message_types_by_name['Metadata'] = _METADATA
+DESCRIPTOR.message_types_by_name['Monorail'] = _MONORAIL
+DESCRIPTOR.enum_types_by_name['OS'] = _OS
+_sym_db.RegisterFileDescriptor(DESCRIPTOR)
+
+Metadata = _reflection.GeneratedProtocolMessageType('Metadata', (_message.Message,), dict(
+  DESCRIPTOR = _METADATA,
+  __module__ = 'directory_metadata_pb2'
+  # @@protoc_insertion_point(class_scope:Metadata)
+  ))
+_sym_db.RegisterMessage(Metadata)
+
+Monorail = _reflection.GeneratedProtocolMessageType('Monorail', (_message.Message,), dict(
+  DESCRIPTOR = _MONORAIL,
+  __module__ = 'directory_metadata_pb2'
+  # @@protoc_insertion_point(class_scope:Monorail)
+  ))
+_sym_db.RegisterMessage(Monorail)
+
+
+# @@protoc_insertion_point(module_scope)