Add a Model to the download component.

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 class DownloadClient {
   // Represents an uninitialized DownloadClient variable.
-  DOWNLOAD_CLIENT_INVALID = 0,
+  INVALID = 0,
 
-  DOWNLOAD_CLIENT_OFFLINE_PAGE_PREFETCH = 1,
+  // Test client values.  Meant to be used by the testing framework and not
+  // production code.  Callers will be unable to access the DownloadService with
+  // these test APIs.
+  TEST_1 = 1,
+  TEST_2 = 2,
+  TEST_3 = 3,

[xingliu, 2017/05/09 06:11:41]

optional nit: this is more like something in tests files. But I don't have
better solution. 

[David Trainor- moved to gerrit, 2017/05/09 15:59:26]

Me neither... I don't want to rely on a client always being there to test this
out.  I could probably just drop it to one test client though...

[Peter Beverloo, 2017/05/10 12:44:45]

We'll have to reflect these in histograms.xml, so if we could reduce this to two
(or one!) clients that'd be grand.

[David Trainor- moved to gerrit, 2017/05/15 15:59:52]

Good point r.e. UMA.  I'll leave it at 1.  If we ever need more than 1 we can
revisit.

 
-  DOWNLOAD_CLIENT_BOUNDARY = 2,
+  OFFLINE_PAGE_PREFETCH = 4,
+
+  BOUNDARY = 5,
 };
 
 }  // namespace download
 
 #endif  // COMPONENTS_DOWNLOAD_PUBLIC_CLIENTS_H_