PrecacheFetcher::CancelPrecaching should run in the main UI thread

components/precache/content/precache_manager.cc

Index: components/precache/content/precache_manager.cc
diff --git a/components/precache/content/precache_manager.cc b/components/precache/content/precache_manager.cc
index 376c4579618c38b8059476dc98f87ce3fcc00689..7457018f6b559b60d967bc13fc82615f61371281 100644
--- a/components/precache/content/precache_manager.cc
+++ b/components/precache/content/precache_manager.cc
@@ -197,12 +197,12 @@ void PrecacheManager::CancelPrecaching() {
   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();
   }