Add the download component and initial setup

components/download/public/client.h

Index: components/download/public/client.h
diff --git a/components/download/public/client.h b/components/download/public/client.h
new file mode 100644
index 0000000000000000000000000000000000000000..081873f5130f94e853a6e5638381571f0f8d328c
--- /dev/null
+++ b/components/download/public/client.h
@@ -0,0 +1,69 @@
+// 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_CLIENT_H_
+#define COMPONENTS_DOWNLOAD_PUBLIC_CLIENT_H_
+
+#include <string>
+#include <vector>
+
+#include "base/files/file_path.h"
+#include "net/http/http_response_headers.h"
+#include "url/gurl.h"
+
+namespace download {
+
+// The Client interface required by any feature that wants to start a download
+// through the BackgroundDownloadService.  Should be registered immediately at
+// startup when the BackgroundDownloadService is created (see the factory).
+class Client {
+ public:
+  // Used by OnDownloadStarted to determine whether or not the DownloadService
+  // should continue downloading the file or abort the attempt.
+  enum ShouldDownload {
+    CONTINUE,
+    ABORT,
+  };
+
+  // Called when the DownloadService is initialized and ready to be interacted
+  // with.  |outstanding_download_guids| is a list of all downloads the
+  // DownloadService is aware of that are associated with this Client.
+  void OnServiceInitialized(

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

Are the `virtual` keywords missing everywhere?

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

O_o

+      const std::vector<std::string>& outstanding_download_guids);

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

= 0?

It'd be good to have either everything be pure virtual, or nothing.

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Done.

+
+  // Return whether or not the download should be aborted (potentially in
+  // response to |headers|).  The download will be downloading at the time this
+  // call is made.
+  ShouldDownload OnDownloadStarted(
+      const std::string& guid,
+      const GURL& final_url,

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

Why not an std::vector<GURL> with the entire URL chain? It's available from
DownloadItem::GetUrlChain() and at least Background Fetch will consume it.

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Done.

+      const scoped_refptr<const net::HttpResponseHeaders> headers) = 0;

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

const&?

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Done.

+
+  void OnDownloadUpdated(const std::string& guid,

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

docs? Especially the frequency would be interesting to know.

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Done.

+                         uint64_t bytes_downloaded) = 0;
+
+  // TODO(dtrainor): Expose a useful error message with the failed download.
+  void OnDownloadFailed(const std::string& guid) = 0;
+
+  // Called when the download was not completed before the
+  // DownloadParams::cancel_after timeout.
+  void OnDownloadTimedOut(const std::string& guid) = 0;
+
+  // Called when the download has been aborted due to a specific number of
+  // retries or a specific number of bytes wasted.

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

"or a specific number of bytes wasted" what does that mean?

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Failing only based on retry count is probably not the best decision.  We could
retry more than the default 3 times if we failed after 10 bytes.  But if we
failed after 1gb we might want to just give up even if we have another chance to
retry.

+  void OnDownloadAborted(const std::string& guid) = 0;
+
+  // Called when a download has been successfully completed.  After this call
+  // the download entry will be purged from the database.  The file will be
+  // automatically removed if it is not renamed or deleted after a window of
+  // time (hours).

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

define "time (hours)"

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Done.

+  // TODO(dtrainor): Investigate alternate output formats.
+  void OnDownloadSucceeded(const std::string& guid,
+                           const base::FilePath& path,
+                           uint64_t size) = 0;
+};

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

+virtual dtor

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Done.

+
+}  // namespace download
+
+#endif  // COMPONENTS_DOWNLOAD_PUBLIC_CLIENT_H_