Fix a bug that causes a DCHECK to sometimes get hit for no good reason.

BUG=584574

Review-Url: https://codereview.chromium.org/2333893002
Cr-Commit-Position: refs/heads/master@{#418740}

content/browser/appcache/appcache_request_handler.cc

@@ -380,7 +380,7 @@ void AppCacheRequestHandler::OnMainResponseFound(
     return;
   }
 
-  if (IsResourceTypeFrame(resource_type_) && cache_id != kAppCacheNoCacheId) {
+  if (IsMainResourceType(resource_type_) && cache_id != kAppCacheNoCacheId) {
     // AppCacheHost loads and holds a reference to the main resource cache
     // for two reasons, firstly to preload the cache into the working set
     // in advance of subresource loads happening, secondly to prevent the