Bug 1361699 - Add buffer when writing hashstore to file r=gcp,mcmanus

We write a lot of 4-bytes prefixes to file which call many system calls.
We should use a buffer and only write to file if the buffer is full or
finish writing. nsIBufferedOutputStream is a good candidate to do that

MozReview-Commit-ID: CzGOd7iXVTv

toolkit/components/url-classifier/HashStore.cpp

@@ -976,8 +976,7 @@ HashStore::WriteFile()
   NS_ENSURE_SUCCESS(rv, rv);
 
   nsCOMPtr<nsIOutputStream> out;
-  rv = NS_NewCheckSummedOutputStream(getter_AddRefs(out), storeFile,
-                                     PR_WRONLY | PR_TRUNCATE | PR_CREATE_FILE);
+  rv = NS_NewCheckSummedOutputStream(getter_AddRefs(out), storeFile);
   NS_ENSURE_SUCCESS(rv, rv);
 
   uint32_t written;