downloads: add the ability to undo download removal.

chrome/browser/download/download_path_reservation_tracker.cc

Index: chrome/browser/download/download_path_reservation_tracker.cc
diff --git a/chrome/browser/download/download_path_reservation_tracker.cc b/chrome/browser/download/download_path_reservation_tracker.cc
index 8ced0eeecf6a9ded4076e6a5d80a36ab2426bcef..1b0476941f53d1bbd474483809e359773e21aba1 100644
--- a/chrome/browser/download/download_path_reservation_tracker.cc
+++ b/chrome/browser/download/download_path_reservation_tracker.cc
@@ -326,6 +326,10 @@ void DownloadItemObserver::OnDownloadUpdated(DownloadItem* download) {
       delete this;
       break;
 
+    case DownloadItem::REMOVED:
+      // This is handled later by Remove().

[Dan Beam, 2014/11/13 02:51:16]

should we change the state in the downloads database?  maybe in the rare event
that the browser crashes or something?  (and ~DownloadsDOMHandler isn't run)

+      break;
+
     case DownloadItem::MAX_DOWNLOAD_STATE:
       // Compiler appeasement.
       NOTREACHED();