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, update the metric DownloadService.DownloadClients in
+// histograms.xml and make sure to keep this list in sync.  Additions should be
+// treated as APPEND ONLY to make sure to keep both UMA metric semantics correct
+// but also to make sure the underlying database properly associates each
+// download with the right client.
+enum DownloadClient {
+  // Represents an uninitialized DownloadClient variable.
+  INVALID = 0,
+
+  OFFLINE_PAGE_PREFETCH = 1,
+
+  BOUNDARY = 2,

[Peter Beverloo, 2017/05/03 13:02:50]

You'd use this as download::BOUNDARY, which means it's super likely to collide
with some other value/enumeration. Using an `enum class` would require
static_cast<>s to get the integral values, so maybe prefix these instead?

DOWNLOAD_CLIENT_INVALID,
DOWNLOAD_CLIENT_OFFLINE_PAGE_PREFETCH,
DOWNLOAD_CLIENT_BOUNDARY

[David Trainor- moved to gerrit, 2017/05/03 17:42:03]

Done.

+};
+
+}  // namespace download
+
+#endif  // COMPONENTS_DOWNLOAD_PUBLIC_CLIENTS_H_