reapply r70628. It was wrongly blamed for breaking Linux valgrind bots.

webkit/appcache/appcache_disk_cache.cc

Index: webkit/appcache/appcache_disk_cache.cc
diff --git a/webkit/appcache/appcache_disk_cache.cc b/webkit/appcache/appcache_disk_cache.cc
index dbba21141402e66792edd2f7ceff6895da6e89ef..9aa0280add56bad5950e1afedee124b49b7c4c53 100644
--- a/webkit/appcache/appcache_disk_cache.cc
+++ b/webkit/appcache/appcache_disk_cache.cc
@@ -108,7 +108,7 @@ int AppCacheDiskCache::Init(net::CacheType cache_type,
       this, &AppCacheDiskCache::OnCreateBackendComplete);
 
   int rv = disk_cache::CreateCacheBackend(
-      cache_type, cache_directory, cache_size, force, cache_thread,
+      cache_type, cache_directory, cache_size, force, cache_thread, NULL,
       &(create_backend_callback_->backend_ptr_), create_backend_callback_);
   if (rv == net::ERR_IO_PENDING)
     init_callback_ = callback;