Move InMemoryURLIndex Caching Operations to FILE Thread

chrome/browser/history/in_memory_history_backend.h

Index: chrome/browser/history/in_memory_history_backend.h
===================================================================
--- chrome/browser/history/in_memory_history_backend.h	(revision 117518)
+++ chrome/browser/history/in_memory_history_backend.h	(working copy)
@@ -69,9 +69,6 @@
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
-  // Return the quick history index.
-  history::InMemoryURLIndex* InMemoryIndex() const { return index_.get(); }
-
  private:
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, DeleteAll);
 
@@ -95,9 +92,6 @@
   // initialization.
   Profile* profile_;
 
-  // The index used for quick history lookups.
-  scoped_ptr<history::InMemoryURLIndex> index_;
-
   DISALLOW_COPY_AND_ASSIGN(InMemoryHistoryBackend);
 };