Fix destruction concurrency issue in PrerenderLocalPredictor:

chrome/browser/prerender/prerender_local_predictor.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/prerender/prerender_local_predictor.h"
 
 #include "base/timer.h"
 #include "chrome/browser/prerender/prerender_manager.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/history/history.h"
@@ skipping 70 matching lines @@
       visit_history_initialized_(false) {
   if (MessageLoop::current()) {
     timer_.Start(FROM_HERE,
                  base::TimeDelta::FromMilliseconds(kInitDelayMs),
                  this,
                  &PrerenderLocalPredictor::Init);
   }
 }
 
 PrerenderLocalPredictor::~PrerenderLocalPredictor() {
-  HistoryService* history = GetHistoryIfExists();
-  if (history)
-    history->RemoveVisitDatabaseObserver(this);
+  if (observing_history_service_.get())
+    observing_history_service_->RemoveVisitDatabaseObserver(this);
 }
 
 void PrerenderLocalPredictor::OnAddVisit(const history::BriefVisitInfo& info) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   HistoryService* history = GetHistoryIfExists();
   if (history) {
     history->ScheduleDBTask(
         new GetURLForURLIDTask(this, info.url_id),
         &history_db_consumer_);
   }
@@ skipping 22 matching lines @@
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   HistoryService* history = GetHistoryIfExists();
   if (!history) {
     // TODO(tburkard): Record this somewhere (eg histogram) and/or try again
     // later.
     return;
   }
   history->ScheduleDBTask(
       new GetVisitHistoryTask(this, kMaxVisitHistory),
       &history_db_consumer_);
-  history->AddVisitDatabaseObserver(this);
+  observing_history_service_ = history;
+  observing_history_service_->AddVisitDatabaseObserver(this);
 }
 
 }  // namespace prerender