Detect removed files and reflect the state in chrome://downloads and the download shelf

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

Index: chrome/browser/ui/gtk/download/download_shelf_gtk.h
diff --git a/chrome/browser/ui/gtk/download/download_shelf_gtk.h b/chrome/browser/ui/gtk/download/download_shelf_gtk.h
index 5d0f1170dec9db1b2ac28a6ebb186ae530a6d92d..56e1eeb1e6c4f1ae4a3b2a2abb6872a422afe5a2 100644
--- a/chrome/browser/ui/gtk/download/download_shelf_gtk.h
+++ b/chrome/browser/ui/gtk/download/download_shelf_gtk.h
@@ -75,6 +75,11 @@ class DownloadShelfGtk : public DownloadShelf,
   // size is changed.
   void MaybeShowMoreDownloadItems();
 
+  // Checks whether downloaded files still exist. Updates state of downloads
+  // that refer to removed files. The check runs in the background and may
+  // finish asynchronously after this method returns.
+  void CheckForFilesRemoval();
+
   // Checks that all download items have been opened, and sets the auto-close
   // state of the shelf if so.
   void AutoCloseIfPossible();