Do no use AppCache after a successful 304 response.

This is a continuation of:
https://chromium-review.googlesource.com/c/chromium/src/+/1503133/5
where the condition before running |MaybeCreateLoaderForResponse| was
removed by mistake. This restores the condition and adds an
explanation about why it is useful.

Bug: 873269
Change-Id: Ieda17fca7baacfeec00468e01eb67332802ee905
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505938Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638180}

content/browser/loader/navigation_url_loader_impl.cc

@@ -1331,11 +1331,17 @@ class NavigationURLLoaderImpl::URLLoaderRequestController
           GetCertificateChainsSizeInKB(status.ssl_info.value()));
     }
 
-    // If the default loader (network) was used to handle the URL load
-    // request we need to see if the interceptors want to potentially create a
-    // new loader for the response. e.g. AppCache.
-    if (MaybeCreateLoaderForResponse(network::ResourceResponseHead()))
+    // If the default loader (network) was used to handle the URL load request
+    // we need to see if the interceptors want to potentially create a new
+    // loader for the response. e.g. AppCache.
+    //
+    // Note: Despite having received a response, the HTTP_NOT_MODIFIED(304) ones
+    //       are ignored using OnComplete(net::ERR_ABORTED). No interceptor must
+    //       be used in this case.
+    if (!received_response_ &&
+        MaybeCreateLoaderForResponse(network::ResourceResponseHead())) {
       return;
+    }
 
     status_ = status;
     base::PostTaskWithTraits(