Make HistoryDBTask not refcounted, and ensure it's destroyed on its origin thread.

chrome/browser/history/in_memory_url_index.cc

Index: chrome/browser/history/in_memory_url_index.cc
diff --git a/chrome/browser/history/in_memory_url_index.cc b/chrome/browser/history/in_memory_url_index.cc
index 7d22dd9f0b2e85e41ec4f8d750922c26d4827746..93cb2c863b0b569973b14bb3e8553cb12126e20e 100644
--- a/chrome/browser/history/in_memory_url_index.cc
+++ b/chrome/browser/history/in_memory_url_index.cc
@@ -296,8 +296,9 @@ void InMemoryURLIndex::ScheduleRebuildFromHistory() {
       HistoryServiceFactory::GetForProfile(profile_,
                                            Profile::EXPLICIT_ACCESS);
   service->ScheduleDBTask(
-      new InMemoryURLIndex::RebuildPrivateDataFromHistoryDBTask(
-          this, languages_, scheme_whitelist_),
+      scoped_ptr<history::HistoryDBTask>(
+          new InMemoryURLIndex::RebuildPrivateDataFromHistoryDBTask(
+              this, languages_, scheme_whitelist_)),
       &cache_reader_tracker_);
 }