Add the ability to unload the HistoryBackend....

chrome/browser/profile.h

Index: chrome/browser/profile.h
===================================================================
--- chrome/browser/profile.h	(revision 28361)
+++ chrome/browser/profile.h	(working copy)
@@ -191,6 +191,10 @@
   // the ServiceAccessType definition above.
   virtual HistoryService* GetHistoryService(ServiceAccessType access) = 0;
 
+  // Similar to GetHistoryService(), but won't create the history service if it
+  // doesn't already exist.
+  virtual HistoryService* GetHistoryServiceWithoutCreating() = 0;
+
   // Returns the WebDataService for this profile. This is owned by
   // the Profile. Callers that outlive the life of this profile need to be
   // sure they refcount the returned value.
@@ -387,6 +391,7 @@
   virtual ExtensionMessageService* GetExtensionMessageService();
   virtual FaviconService* GetFaviconService(ServiceAccessType sat);
   virtual HistoryService* GetHistoryService(ServiceAccessType sat);
+  virtual HistoryService* GetHistoryServiceWithoutCreating();
   virtual WebDataService* GetWebDataService(ServiceAccessType sat);
   virtual PasswordStore* GetPasswordStore(ServiceAccessType sat);
   virtual PrefService* GetPrefs();