downloads: add the ability to undo download removal.

content/browser/download/download_item_impl.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.
 
 // File method ordering: Methods in this file are in the same order as
 // in download_item_impl.h, with the following exception: The public
 // interface Start is placed in chronological order with the other
 // (private) routines that together define a DownloadItem's state
 // transitions as the download progresses.  See "Download progression
 // cascade" later in this file.
@@ skipping 323 matching lines @@
     BrowserThread::PostTaskAndReplyWithResult(
         BrowserThread::FILE,
         FROM_HERE,
         base::Bind(&DownloadFileDetach, base::Passed(&download_file_)),
         callback);
   } else {
     callback.Run(current_path_);
   }
   current_path_.clear();
   Remove();
-  // We have now been deleted.

[benjhayden, 2014/11/21 00:50:00]

I found these comments helpful. Why remove them?

[Dan Beam, 2014/11/21 05:32:21]

because (with this change) they're no longer correct

 }
 
 void DownloadItemImpl::Pause() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   // Ignore irrelevant states.
   if (state_ != IN_PROGRESS_INTERNAL || is_paused_)
     return;
 
   request_handle_->PauseRequest();
@@ skipping 81 matching lines @@
 void DownloadItemImpl::Remove() {
   VLOG(20) << __FUNCTION__ << "() download = " << DebugString(true);
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   delegate_->AssertStateConsistent(this);
   Cancel(true);
   delegate_->AssertStateConsistent(this);
 
   NotifyRemoved();
   delegate_->DownloadRemoved(this);
-  // We have now been deleted.
 }
 
 void DownloadItemImpl::OpenDownload() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   if (!IsDone()) {
     // We don't honor the open_when_complete_ flag for temporary
     // downloads. Don't set it because it shows up in the UI.
     if (!IsTemporary())
       open_when_complete_ = !open_when_complete_;
@@ skipping 1346 matching lines @@
     case RESUME_MODE_USER_CONTINUE:
       return "USER_CONTINUE";
     case RESUME_MODE_USER_RESTART:
       return "USER_RESTART";
   }
   NOTREACHED() << "Unknown resume mode " << mode;
   return "unknown";
 }
 
 }  // namespace content