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 58d13cf566e0818e597721f22682e110de359de0..38a5ec6a995a88f85e707133fa122866a34d4dd6 100644
--- a/chrome/browser/download/download_manager.h
+++ b/chrome/browser/download/download_manager.h
@@ -235,6 +235,16 @@ 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 CheckForHistoryFilesRemoval();
+
+  // Checks whether a downloaded file still exists and updates the file's state
+  // if the file is already removed. The check runs in the background and may
+  // finish asynchronously after this method returns.
+  void CheckForFileRemoval(DownloadItem* download_item);
+
   // Callback function after url is checked with safebrowsing service.
   void CheckDownloadUrlDone(int32 download_id, bool is_dangerous_url);
 
@@ -283,6 +293,14 @@ class DownloadManager
 
   virtual ~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.