PrecacheFetcher::CancelPrecaching should run in the main UI thread

components/precache/content/precache_manager.cc

 // Copyright 2013 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 "components/precache/content/precache_manager.h"
 
 #include <string>
 #include <utility>
 #include <vector>
 
@@ skipping 179 matching lines @@
     // Do nothing if precaching is not in progress.
     return;
   }
   is_precaching_ = false;
   // If cancellation occurs after StartPrecaching but before OnHostsReceived,
   // is_precaching will be true, but the precache_fetcher_ will not yet be
   // constructed.
   if (precache_fetcher_) {
     std::unique_ptr<PrecacheUnfinishedWork> unfinished_work =
         precache_fetcher_->CancelPrecaching();
-    BrowserThread::PostTask(
-        BrowserThread::DB,
-        FROM_HERE,
-        base::Bind(&PrecacheDatabase::SaveUnfinishedWork,
-                   precache_database_->GetWeakPtr(),
-                   base::Passed(&unfinished_work)));
+    if (unfinished_work) {
+      BrowserThread::PostTask(BrowserThread::DB, FROM_HERE,
+                              base::Bind(&PrecacheDatabase::SaveUnfinishedWork,
+                                         precache_database_->GetWeakPtr(),
+                                         base::Passed(&unfinished_work)));
+    }
     // Destroying the |precache_fetcher_| will cancel any fetch in progress.
     precache_fetcher_.reset();
   }
   precache_completion_callback_.Reset();
 }
 
 bool PrecacheManager::IsPrecaching() const {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   return is_precaching_;
 }
@@ skipping 112 matching lines @@
       this));
   precache_fetcher_->Start();
 }
 
 void PrecacheManager::OnHostsReceivedThenDone(
     const history::TopHostsList& host_counts) {
   OnDone();
 }
 
 }  // namespace precache