downloads: add the ability to undo download removal.

content/browser/download/download_item_impl.h

Index: content/browser/download/download_item_impl.h
diff --git a/content/browser/download/download_item_impl.h b/content/browser/download/download_item_impl.h
index b6394f7469dc777805772add0fa46b4177995171..e301b56a56cd1197281f0d982461e687ae994673 100644
--- a/content/browser/download/download_item_impl.h
+++ b/content/browser/download/download_item_impl.h
@@ -97,6 +97,8 @@ class CONTENT_EXPORT DownloadItemImpl
   void Pause() override;
   void Resume() override;
   void Cancel(bool user_cancel) override;
+  void MarkRemoved() override;
+  void UndoRemove() override;
   void Remove() override;
   void OpenDownload() override;
   void ShowDownloadInShell() override;
@@ -281,6 +283,17 @@ class CONTENT_EXPORT DownloadItemImpl
     //   CANCELLED_INTERNAL    On cancel.
     RESUMING_INTERNAL,
 
+    // The user removed this download from history but can undo this action.
+    // Transitions from:
+    //   CANCELLED_INTERNAL
+    //   COMPLETE_INTERNAL
+    //   INTERRUPTED_INTERNAL
+    // Transitions to:
+    //   CANCELLED_INTERNAL
+    //   COMPLETE_INTERNAL
+    //   INTERRUPTED_INTERNAL
+    REMOVED_INTERNAL,
+
     MAX_DOWNLOAD_INTERNAL_STATE,
   };
 
@@ -489,6 +502,9 @@ class CONTENT_EXPORT DownloadItemImpl
   // The current state of this download.
   DownloadInternalState state_;
 
+  // The previous state of this download (set in TransitionTo()).
+  DownloadInternalState prev_state_;
+
   // Current danger type for the download.
   DownloadDangerType danger_type_;