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
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+++ b/components/download/public/client.h
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+// 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,
+  };
+
+  virtual ~Client() = default;
+
+  // 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.
+  virtual void OnServiceInitialized(
+      const std::vector<std::string>& outstanding_download_guids) = 0;
+
+  // 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.
+  virtual ShouldDownload OnDownloadStarted(
+      const std::string& guid,
+      const std::vector<GURL>& url_chain,
+      const scoped_refptr<const net::HttpResponseHeaders>& headers) = 0;
+
+  // Will be called when there is an update to the current progress state of the
+  // underlying download.  Note that |bytes_downloaded| may go backwards if the
+  // download had to be started over from the beginning due to an interruption.
+  // This will be called frequently if the download is actively downloading,
+  // with byte updates coming in as they are processed by the internal download
+  // driver.
+  virtual void OnDownloadUpdated(const std::string& guid,
+                                 uint64_t bytes_downloaded) = 0;
+
+  // TODO(dtrainor): Expose a useful error message with the failed download.
+  virtual void OnDownloadFailed(const std::string& guid) = 0;
+
+  // Called when the download was not completed before the
+  // DownloadParams::cancel_after timeout.
+  virtual void OnDownloadTimedOut(const std::string& guid) = 0;
+
+  // Called when the download has been aborted after reaching a treshold where
+  // we decide it is not worth attempting to start again.  This could be either
+  // due to a specific number of failed retry attempts or a specific number of
+  // wasted bytes due to the download restarting.
+  virtual 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 (12 hours, but finch configurable).  The timeout is meant to be a
+  // failsafe to ensure that we clean up properly.
+  // TODO(dtrainor): Investigate alternate output formats.
+  // TODO(dtrainor): Point to finch configurable timeout when it is added.
+  virtual void OnDownloadSucceeded(const std::string& guid,
+                                   const base::FilePath& path,
+                                   uint64_t size) = 0;
+};
+
+}  // namespace download
+
+#endif  // COMPONENTS_DOWNLOAD_PUBLIC_CLIENT_H_