downloads: add the ability to undo download removal.

chrome/browser/download/download_item_model.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_item_model.h"
 
 #include "base/i18n/number_formatting.h"
 #include "base/i18n/rtl.h"
 #include "base/metrics/field_trial.h"
 #include "base/strings/string16.h"
@@ skipping 515 matching lines @@
       // If the download completed, then rely on GetAutoOpened() to check for
       // opening behavior. This should accurately reflect whether the download
       // was successfully opened.  Extensions, for example, may fail to open.
       return download_->GetAutoOpened() || download_->IsTemporary();
 
     case DownloadItem::CANCELLED:
     case DownloadItem::INTERRUPTED:
       // Interrupted or cancelled downloads should remain on the shelf.
       return false;
 
+    case DownloadItem::REMOVED:

[benjhayden, 2014/11/13 18:01:59]

How do you ensure that this isn't reached?

     case DownloadItem::MAX_DOWNLOAD_STATE:
       NOTREACHED();
   }
 
   NOTREACHED();
   return false;
 }
 
 bool DownloadItemModel::ShouldShowDownloadStartedAnimation() const {
   return !download_->IsSavePackageDownload() &&
@@ skipping 147 matching lines @@
   if (!download_service)
     return;
 
   ChromeDownloadManagerDelegate* delegate =
       download_service->GetDownloadManagerDelegate();
   if (!delegate)
     return;
   delegate->OpenDownloadUsingPlatformHandler(download_);
   RecordDownloadOpenMethod(DOWNLOAD_OPEN_METHOD_USER_PLATFORM);
 }