Add the download component and initial setup

components/download/public/clients.h

+// Copyright 2017 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.
+
+#ifndef COMPONENTS_DOWNLOAD_PUBLIC_CLIENTS_H_
+#define COMPONENTS_DOWNLOAD_PUBLIC_CLIENTS_H_
+
+namespace download {
+
+// A list of all clients that are able to make download requests through the
+// DownloadService.
+// To add a new client, add the metric here and update
+// DownloadService.DownloadClients in histograms.xml.

[Peter Beverloo, 2017/05/02 16:12:21]

nit: mention that it's append only and that existing values should not be
changed.

[David Trainor- moved to gerrit, 2017/05/03 06:02:59]

Will do.  Histogram isn't added yet since there is no metric that will actually
touch it.  Will add that the moment we add the underlying DownloadService
implementation.

+enum DownloadClient {
+  // Represents an uninitialized DownloadClient variable.
+  INVALID = 0,
+
+  OFFLINE_PAGE_PREFETCH = 1,
+
+  BOUNDARY = 2,
+};
+
+}  // namespace download
+
+#endif  // COMPONENTS_DOWNLOAD_PUBLIC_CLIENTS_H_