Only create one downloads tab per window. If there is already a Downloads...

chrome/browser/download/download_shelf.cc

 // Copyright (c) 2009 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_shelf.h"
 
 #include "base/file_util.h"
 #include "chrome/browser/dom_ui/downloads_ui.h"
 #include "chrome/browser/download/download_item_model.h"
 #include "chrome/browser/download/download_manager.h"
 #include "chrome/browser/metrics/user_metrics.h"
 #include "chrome/browser/tab_contents/tab_contents.h"
 #include "chrome/common/l10n_util.h"
 #include "chrome/common/url_constants.h"
 #include "grit/generated_resources.h"
 
 #if defined(OS_WIN)
 #include "chrome/browser/download/download_util.h"
 #elif defined(OS_POSIX)
 #include "chrome/common/temp_scaffolding_stubs.h"
 #endif
 
 // DownloadShelf ---------------------------------------------------------------
 
 void DownloadShelf::ShowAllDownloads() {
   Profile* profile = tab_contents_->profile();
   if (profile)
     UserMetrics::RecordAction(L"ShowDownloads", profile);
   tab_contents_->OpenURL(GURL(chrome::kChromeUIDownloadsURL), GURL(),
-                         NEW_FOREGROUND_TAB, PageTransition::AUTO_BOOKMARK);
+                         SINGLETON_TAB, PageTransition::AUTO_BOOKMARK);
 }
 
 void DownloadShelf::ChangeTabContents(TabContents* old_contents,
                                       TabContents* new_contents) {
   DCHECK(old_contents == tab_contents_);
   tab_contents_ = new_contents;
 }
 
 // DownloadShelfContextMenu ----------------------------------------------------
 
@@ skipping 81 matching lines @@
           extension, !ItemIsChecked(ALWAYS_OPEN_TYPE));
       break;
     }
     case CANCEL:
       model_->CancelTask();
       break;
     default:
       NOTREACHED();
   }
 }