downloads: add the ability to undo download removal.

content/public/browser/download_manager.h

Index: content/public/browser/download_manager.h
diff --git a/content/public/browser/download_manager.h b/content/public/browser/download_manager.h
index d51ebe9b294d37549d1bf43a7fc750dd820efd97..a03e54baeed79ae73159600729bc401ad6c0ab6c 100644
--- a/content/public/browser/download_manager.h
+++ b/content/public/browser/download_manager.h
@@ -83,6 +83,10 @@ class CONTENT_EXPORT DownloadManager : public base::SupportsUserData::Data {
     virtual void OnDownloadCreated(
         DownloadManager* manager, DownloadItem* item) {}
 
+    // Happens when a removed item is revived (the removal is undone).
+    virtual void OnDownloadRevived(
+        DownloadManager* manager, DownloadItem* item) {}
+
     // A SavePackage has successfully finished.
     virtual void OnSavePackageSuccessfullyFinished(
         DownloadManager* manager, DownloadItem* item) {}
@@ -111,20 +115,25 @@ class CONTENT_EXPORT DownloadManager : public base::SupportsUserData::Data {
       const DownloadUrlParameters::OnStartedCallback& on_started) = 0;
 
   // Remove downloads after remove_begin (inclusive) and before remove_end
-  // (exclusive). You may pass in null Time values to do an unbounded delete
+  // (exclusive). You may pass in null Time values to do an unbounded removal
   // in either direction.
   virtual int RemoveDownloadsBetween(base::Time remove_begin,
                                      base::Time remove_end) = 0;
 
-  // Remove downloads will delete all downloads that have a timestamp that is
-  // the same or more recent than |remove_begin|. The number of downloads
-  // deleted is returned back to the caller.
+  // Remove all downloads that have a timestamp that is the same or more recent
+  // than |remove_begin|. The number of downloads deleted is returned back to
+  // the caller.
   virtual int RemoveDownloads(base::Time remove_begin) = 0;
 
-  // Remove all downloads will delete all downloads. The number of downloads
-  // deleted is returned back to the caller.
+  // Remove all downloads. Returns the number of downloads removed.
   virtual int RemoveAllDownloads() = 0;
 
+  // Revives a removed download (undoes the removal).
+  virtual void ReviveDownload(uint32 id) = 0;
+
+  // Finalizes removal of a download. Downloads cannot be revived after this.
+  virtual void FinalizeRemoval(uint32 id) = 0;
+
   // See DownloadUrlParameters for details about controlling the download.
   virtual void DownloadUrl(scoped_ptr<DownloadUrlParameters> parameters) = 0;