--downloads-new-ui completely disables the download shelf.

chrome/browser/about_flags.cc

 // Copyright (c) 2011 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/about_flags.h"
 
 #include <algorithm>
 #include <iterator>
 #include <map>
 #include <set>
@@ skipping 353 matching lines @@
     "memory-widget",
     IDS_FLAGS_MEMORY_WIDGET_NAME,
     IDS_FLAGS_MEMORY_WIDGET_DESCRIPTION,
     kOsCrOS,
 #if defined(OS_CHROMEOS)
     // This switch exists only on Chrome OS.
     SINGLE_VALUE_TYPE(switches::kMemoryWidget)
 #else
     SINGLE_VALUE_TYPE("")
 #endif
-  },};
+  },
+  {
+    "downloads-new-ui",  // FLAGS:RECORD_UMA
+    IDS_FLAGS_DOWNLOADS_NEW_UI_NAME,
+    IDS_FLAGS_DOWNLOADS_NEW_UI_DESCRIPTION,
+    kOsAll,
+    SINGLE_VALUE_TYPE(switches::kDownloadsNewUI)
+  },
+};
 
 const Experiment* experiments = kExperiments;
 size_t num_experiments = arraysize(kExperiments);
 
 // Stores and encapsulates the little state that about:flags has.
 class FlagsState {
  public:
   FlagsState() : needs_restart_(false) {}
   void ConvertFlagsToSwitches(PrefService* prefs, CommandLine* command_line);
   bool IsRestartNeededToCommitChanges();
@@ skipping 419 matching lines @@
 }
 
 const Experiment* GetExperiments(size_t* count) {
   *count = num_experiments;
   return experiments;
 }
 
 }  // namespace testing
 
 }  // namespace about_flags