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

chrome/app/generated_resources.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!--
 This file contains definitions of resources that will be translated for each
 locale.  The variables is_win, is_macosx, is_linux, and is_posix are available
 for making strings OS specific.  Other platform defines such as use_titlecase
 are declared in build/common.gypi.
 -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
@@ skipping 4215 matching lines @@
       </message>
       <message name="IDS_FLAGS_ENABLE_SHORTCUTS_PROVIDER_DESCRIPTION" desc=" Description of the 'Enable better ranking of previously selected shortcuts in omnibox' lab.">
         Remembers commonly selected autocomplete results based on the omnibox search term and offers those URLs the next time that same search term is typed.
       </message>      
       <message name="IDS_FLAGS_MEMORY_WIDGET_NAME" desc="Name of the 'Show memory status' lab.">
         Show memory status
       </message>
       <message name="IDS_FLAGS_MEMORY_WIDGET_DESCRIPTION" desc="Description of the 'Show memory status' lab.">
         Shows the amount of free memory in the status area. Click on the menu for more information about system memory.
       </message>      
+      <message name="IDS_FLAGS_DOWNLOADS_NEW_UI_NAME" desc="Name of the 'New Downloads UI' lab.">
+        New Downloads UI
+      </message>
+      <message name="IDS_FLAGS_DOWNLOADS_NEW_UI_DESCRIPTION" desc="Description of the 'New Downloads UI' lab.">
+        Shows an experimental new UI instead of the download shelf.
+      </message>
       <message name="IDS_FLAGS_XSS_AUDITOR_NAME" desc="Description of the 'XSS Auditor' lab.">
         XSS Auditor
       </message>
       <message name="IDS_FLAGS_XSS_AUDITOR_DESCRIPTION" desc="Description of the 'XSS Auditor' lab.">
         Enables WebKit's XSS Auditor (cross-site scripting protection). This feature aims to protect you from certain attacks of malicious web sites. It improves your security, but it might not be compatible with all web sites.
       </message>
       <message name="IDS_FLAGS_CLOUD_PRINT_PROXY_NAME" desc="Title of the Cloud Print Proxy lab">
         Cloud Print Proxy
       </message>
       <message name="IDS_FLAGS_CLOUD_PRINT_PROXY_DESCRIPTION" desc="Description of the Cloud Print Proxy lab">
@@ skipping 9130 matching lines @@
     </includes>
     <structures fallback_to_english="true">
       <structure name="IDD_CHROME_FRAME_FIND_DIALOG" file="cf_resources.rc" type="dialog" >
       </structure>
       <structure name="IDD_CHROME_FRAME_READY_PROMPT" file="cf_resources.rc" type="dialog" >
       </structure>
     </structures>
 
   </release>
 </grit>