Add new UMA stats to get a handle on Downloads UI Usage

chrome/browser/ui/gtk/download/download_shelf_gtk.cc

Index: chrome/browser/ui/gtk/download/download_shelf_gtk.cc
diff --git a/chrome/browser/ui/gtk/download/download_shelf_gtk.cc b/chrome/browser/ui/gtk/download/download_shelf_gtk.cc
index 8355c95bf7a96e9e93537c2d501e109357f73a3a..012b887a9d34e9ebb9ea0666405ed3af290fedb4 100644
--- a/chrome/browser/ui/gtk/download/download_shelf_gtk.cc
+++ b/chrome/browser/ui/gtk/download/download_shelf_gtk.cc
@@ -18,6 +18,7 @@
 #include "chrome/browser/ui/gtk/gtk_util.h"
 #include "chrome/common/chrome_notification_types.h"
 #include "content/browser/download/download_item.h"
+#include "content/browser/download/download_stats.h"
 #include "content/common/notification_service.h"
 #include "grit/generated_resources.h"
 #include "grit/theme_resources.h"
@@ -194,6 +195,7 @@ void DownloadShelfGtk::Close() {
   gdk_window_raise(shelf_.get()->window);
   slide_widget_->Close();
   browser_->UpdateDownloadShelfVisibility(false);
+  download_stats::RecordShelfSize(download_items_.size());
   SetCloseOnMouseOut(false);
 }