Bug 1325888 - Clear cached completion result when close safebrowsing database. r=francois

Safebrowsing testcase will close and then re-open database to simulate firefox restart.
This bug happened when DB service already has a previously cached completion result and then
re-open database. In this scenario, if testcase triggers another the same completion request,
we won't cache it because we thought we have it already (See nsUrlClassifierDBServiceWorker::CacheCompletion).

This causes a testcase assertion when testcase expects the completion
result should be stored in LookupCache.

MozReview-Commit-ID: 8o57jHv92OH

--HG--
extra : rebase_source : 90861060437d6cef860b85dd669d76e74ceec660

toolkit/components/url-classifier/nsUrlClassifierDBService.cpp

@@ -812,6 +812,12 @@ nsUrlClassifierDBServiceWorker::CloseDb()
     mClassifier = nullptr;
   }
 
+  // Clear last completion result when close db so we will still cache completion
+  // result next time we re-open it.
+  if (mLastResults) {
+    mLastResults->Clear();
+  }
+
   LOG(("urlclassifier db closed\n"));
 
   return NS_OK;