Add the ability to unload the HistoryBackend....

chrome/browser/profile.cc

Index: chrome/browser/profile.cc
===================================================================
--- chrome/browser/profile.cc	(revision 28361)
+++ chrome/browser/profile.cc	(working copy)
@@ -305,6 +305,10 @@
     }
   }
 
+  virtual HistoryService* GetHistoryServiceWithoutCreating() {
+    return profile_->GetHistoryServiceWithoutCreating();
+  }
+
   virtual FaviconService* GetFaviconService(ServiceAccessType sat) {
     if (sat == EXPLICIT_ACCESS) {
       return profile_->GetFaviconService(sat);
@@ -1016,6 +1020,10 @@
   return history_service_.get();
 }
 
+HistoryService* ProfileImpl::GetHistoryServiceWithoutCreating() {
+  return history_service_.get();
+}
+
 TemplateURLModel* ProfileImpl::GetTemplateURLModel() {
   if (!template_url_model_.get())
     template_url_model_.reset(new TemplateURLModel(this));