downloads: add the ability to undo download removal.

chrome/browser/download/download_path_reservation_tracker.cc

 // 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.
 
 #include "chrome/browser/download/download_path_reservation_tracker.h"
 
 #include <map>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 308 matching lines @@
     case DownloadItem::INTERRUPTED:
       // The download filename will need to be re-generated when the download is
       // restarted. Holding on to the reservation now would prevent the name
       // from being used for a subsequent retry attempt.
 
       BrowserThread::PostTask(BrowserThread::FILE, FROM_HERE, base::Bind(
           &RevokeReservation, download));
       delete this;
       break;
 
+    case DownloadItem::REMOVED:
+      // This is handled later by Remove().

[Dan Beam, 2014/11/13 02:51:16]

should we change the state in the downloads database?  maybe in the rare event
that the browser crashes or something?  (and ~DownloadsDOMHandler isn't run)

+      break;
+
     case DownloadItem::MAX_DOWNLOAD_STATE:
       // Compiler appeasement.
       NOTREACHED();
   }
 }
 
 void DownloadItemObserver::OnDownloadDestroyed(DownloadItem* download) {
   // Items should be COMPLETE/INTERRUPTED/CANCELLED before being destroyed.
   NOTREACHED();
   BrowserThread::PostTask(BrowserThread::FILE, FROM_HERE, base::Bind(
@@ skipping 31 matching lines @@
       base::Bind(&RunGetReservedPathCallback,
                  callback,
                  base::Owned(reserved_path)));
 }
 
 // static
 bool DownloadPathReservationTracker::IsPathInUseForTesting(
     const base::FilePath& path) {
   return IsPathInUse(path);
 }