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

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index e1c6d0da4693e521488cb2301f8723029dcd604b..3e35497d9c34fcd3c1efe9a7a57d429b7607b9dc 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -232,6 +232,9 @@ const char kDisableCustomProtocolOSCheck[]  =
 // Disables retrieval of PAC URLs from DHCP as per the WPAD standard.
 const char kDisableDhcpWpad[]               = "disable-dhcp-wpad";
 
+// Disables the download shelf.
+const char kDisableDownloadShelf[]          = "disable-download-shelf";
+
 // Disable extensions.
 const char kDisableExtensions[]             = "disable-extensions";