downloads: add the ability to undo download removal.

chrome/browser/download/download_history.cc

Index: chrome/browser/download/download_history.cc
diff --git a/chrome/browser/download/download_history.cc b/chrome/browser/download/download_history.cc
index 9e960824d6ea007b7b4a6d3db06d8268c5263d66..0d6da87e7b51e65d955fca19f58e0a573ab74f7a 100644
--- a/chrome/browser/download/download_history.cc
+++ b/chrome/browser/download/download_history.cc
@@ -71,7 +71,8 @@ class DownloadHistoryData : public base::SupportsUserData::Data {
 
   explicit DownloadHistoryData(content::DownloadItem* item)
       : state_(NOT_PERSISTED),
-        was_restored_from_history_(false) {
+        was_restored_from_history_(false),
+        was_revived_(false) {
     item->SetUserData(kKey, this);
   }
 
@@ -85,6 +86,9 @@ class DownloadHistoryData : public base::SupportsUserData::Data {
     was_restored_from_history_ = value;
   }
 
+  bool was_revived() const { return was_revived_; }
+  void set_was_revived(bool revived) { was_revived_ = revived; }
+
   // This allows DownloadHistory::OnDownloadUpdated() to see what changed in a
   // DownloadItem if anything, in order to prevent writing to the database
   // unnecessarily.  It is nullified when the item is no longer in progress in
@@ -103,6 +107,7 @@ class DownloadHistoryData : public base::SupportsUserData::Data {
   PersistenceState state_;
   scoped_ptr<history::DownloadRow> info_;
   bool was_restored_from_history_;
+  bool was_revived_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadHistoryData);
 };
@@ -372,8 +377,12 @@ void DownloadHistory::OnDownloadCreated(
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 
   // All downloads should pass through OnDownloadCreated exactly once.
-  CHECK(!DownloadHistoryData::Get(item));
-  DownloadHistoryData* data = new DownloadHistoryData(item);
+  DownloadHistoryData* data = DownloadHistoryData::Get(item);
+  CHECK(!data || data->was_revived());
+
+  if (!data)
+    data = new DownloadHistoryData(item);
+
   if (item->GetId() == loading_id_) {
     data->SetState(DownloadHistoryData::PERSISTED);
     data->set_was_restored_from_history(true);
@@ -385,6 +394,15 @@ void DownloadHistory::OnDownloadCreated(
   MaybeAddToHistory(item);
 }
 
+void DownloadHistory::OnDownloadRevived(
+    content::DownloadManager* manager, content::DownloadItem* item) {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+
+  DownloadHistoryData* data = DownloadHistoryData::Get(item);
+  if (data)
+    data->set_was_revived(true);
+}
+
 void DownloadHistory::OnDownloadUpdated(
     content::DownloadManager* manager, content::DownloadItem* item) {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));