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

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 13a233a999e49071d874281864e4c0186d252958..97fc33a4fb200b5e7ece70097a0dd83b13173ca1 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -364,7 +364,15 @@ const Experiment kExperiments[] = {
 #else
     SINGLE_VALUE_TYPE("")
 #endif
-  },};
+  },
+  {
+    "disable-download-shelf",  // FLAGS:RECORD_UMA
+    IDS_FLAGS_DISABLE_DOWNLOAD_SHELF_NAME,
+    IDS_FLAGS_DISABLE_DOWNLOAD_SHELF_DESCRIPTION,
+    kOsAll - kOsCrOS,

[cbentzel, 2011/08/10 11:54:54]

Nit: Maybe it's just me, but it feels weird to do this instead of

(kOsAll & ~kOsCros)

[benjhayden, 2011/08/10 17:09:17]

Even better, kOsAll.
If we're reimplementing the UI as an extension, then we can be consistent even
across cros.

+    SINGLE_VALUE_TYPE(switches::kDisableDownloadShelf)
+  },
+};
 
 const Experiment* experiments = kExperiments;
 size_t num_experiments = arraysize(kExperiments);