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

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 86face9f544b220fd1c526b1989ffbeda418e8e4..fc2a63e9a00a308fbd28b7c3bc841d15d5210456 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -4227,6 +4227,12 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <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_DISABLE_DOWNLOAD_SHELF_NAME" desc="Title for the flag to disable the download shelf.">
+        Disable the Download Shelf
+      </message>
+      <message name="IDS_FLAGS_DISABLE_DOWNLOAD_SHELF_DESCRIPTION" desc="Description for the flag to disable the download shelf.">
+        Never show the download shelf. You must open chrome://downloads in order to monitor and manage downloads, including catching interrupted and dangerous downloads.
+      </message>
       <message name="IDS_FLAGS_XSS_AUDITOR_NAME" desc="Description of the 'XSS Auditor' lab.">
         XSS Auditor
       </message>