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

chrome/browser/download/download_manager.h

Index: chrome/browser/download/download_manager.h
diff --git a/chrome/browser/download/download_manager.h b/chrome/browser/download/download_manager.h
index 84b83990bdc9d8b103e2cf6de26f95b2a6239305..a22973fa97b01859a6d355f4c94e9c981d838a27 100644
--- a/chrome/browser/download/download_manager.h
+++ b/chrome/browser/download/download_manager.h
@@ -233,6 +233,11 @@ class DownloadManager
   // Called when the user has validated the download of a dangerous file.
   void DangerousDownloadValidated(DownloadItem* download);
 
+  // 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();
+
   // Callback function after url is checked with safebrowsing service.
   void CheckDownloadUrlDone(DownloadCreateInfo* info, bool is_dangerous_url);
 
@@ -272,6 +277,14 @@ class DownloadManager
 
   ~DownloadManager();
 
+  // Called on the FILE thread to check the existence of a downloaded file.
+  void CheckForFileRemovalOnFileThread(int64 db_handle, const FilePath& path);
+
+  // Called on the UI thread if the FILE thread detects the removal of
+  // the downloaded file. The UI thread updates the state of the file
+  // and then notifies this update to the file's observer.
+  void OnFileRemovalDetected(int64 db_handle);
+
   // Called on the download thread to check whether the suggested file path
   // exists.  We don't check if the file exists on the UI thread to avoid UI
   // stalls from interacting with the file system.