downloads: add the ability to undo download removal.

content/public/browser/download_item.h

Index: content/public/browser/download_item.h
diff --git a/content/public/browser/download_item.h b/content/public/browser/download_item.h
index ed6cd3eea4b819fdca161b781ce742ca0ea5be27..b64bd0f71306e8cea3112f7cfc703d065007967c 100644
--- a/content/public/browser/download_item.h
+++ b/content/public/browser/download_item.h
@@ -63,6 +63,9 @@ class CONTENT_EXPORT DownloadItem : public base::SupportsUserData {
     // This state indicates that the download has been interrupted.
     INTERRUPTED,
 
+    // The user removed this download from their history.

[benjhayden, 2014/11/13 18:01:59]

Clarify that the state is not terminal?
"The user removed this download from history but may undo the removal."?

+    REMOVED,
+
     // Maximum value.
     MAX_DOWNLOAD_STATE
   };
@@ -138,6 +141,14 @@ class CONTENT_EXPORT DownloadItem : public base::SupportsUserData {
   // when resuming a download (assuming the server supports byte ranges).
   virtual void Cancel(bool user_cancel) = 0;
 
+  // Marks the download as removed but doesn't actually delete it yet. This
+  // behavior is used from chrome://downloads so removal can be undone.
+  virtual void MarkRemoved() = 0;
+
+  // Undoes the effects of MarkRemoved() and restores the download's |status_|

[benjhayden, 2014/11/13 18:01:59]

This is an interface, so let's say something like "restores the value of
GetState() to its prior value" to avoid implementation details?

+  // to |prev_status_|. Used from chrome://downloads so removal can be undone.
+  virtual void UndoRemove() = 0;
+
   // Removes the download from the views and history. If the download was
   // in-progress or interrupted, then the intermediate file will also be
   // deleted.