[Downloads] Introduce GUIDs for downloads.

content/public/browser/download_item.h

 // Copyright (c) 2012 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.
 //
 // Each download is represented by a DownloadItem, and all DownloadItems
 // are owned by the DownloadManager which maintains a global list of all
 // downloads. DownloadItems are created when a user initiates a download,
 // and exist for the duration of the browser life time.
 //
 // Download observers:
@@ skipping 136 matching lines @@
 
   // Open the file associated with this download.  If the download is
   // still in progress, marks the download to be opened when it is complete.
   virtual void OpenDownload() = 0;
 
   // Show the download via the OS shell.
   virtual void ShowDownloadInShell() = 0;
 
   // State accessors -----------------------------------------------------------
 
+  // Retrieve the ID for this download. The ID is provided by the owner of the
+  // DownloadItem and is expected to uniquely identify the download within the
+  // context of its container during the lifetime of the download.
   virtual uint32_t GetId() const = 0;
+
+  // Retrieve the GUID for this download. The returned string is never empty and
+  // will satisfy base::IsValidGUID(), in addition to uniquely identifying the
+  // download during its lifetime regardless of its container.
+  virtual const std::string& GetGuid() const = 0;
+
+  // Get the current state of the download. See DownloadState for descriptions
+  // of each download state.
   virtual DownloadState GetState() const = 0;
 
   // Returns the most recent interrupt reason for this download. Returns
   // DOWNLOAD_INTERRUPT_REASON_NONE if there is no previous interrupt reason.
   // Cancelled downloads return DOWNLOAD_INTERRUPT_REASON_USER_CANCELLED. If
   // the download was resumed, then the return value is the interrupt reason
   // prior to resumption.
   virtual DownloadInterruptReason GetLastReason() const = 0;
 
   virtual bool IsPaused() const = 0;
@@ skipping 163 matching lines @@
   // return |name|. Has no effect on the final target filename.
   virtual void SetDisplayName(const base::FilePath& name) = 0;
 
   // Debug/testing -------------------------------------------------------------
   virtual std::string DebugString(bool verbose) const = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_DOWNLOAD_ITEM_H_