Fix pages with feed mime types not being rendered as plain text with network service.

This matches the old path's mime_sniffing_resource_handler.cc.

Bug: 729849
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I7afd5ac3d595ddcea5b061dd064e4413eab590df
Reviewed-on: https://chromium-review.googlesource.com/1173764Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582916}

services/network/url_loader.cc

@@ -701,8 +701,14 @@ void URLLoader::OnResponseStarted(net::URLRequest* url_request, int net_error) {
       ShouldSniffContent(url_request_.get(), response_.get())) {
     is_more_mime_sniffing_needed_ = true;
   }
-  if (!is_more_mime_sniffing_needed_ && !is_more_corb_sniffing_needed_)
+  if (!is_more_mime_sniffing_needed_ && !is_more_corb_sniffing_needed_) {
+    // Treat feed types as text/plain.
+    if (response_->head.mime_type == "application/rss+xml" ||
+        response_->head.mime_type == "application/atom+xml") {
+      response_->head.mime_type.assign("text/plain");
+    }
     SendResponseToClient();
+  }
 
   // Start reading...
   ReadMore();

third_party/WebKit/LayoutTests/FlagExpectations/enable-features=NetworkService

@@ -7,8 +7,6 @@ crbug.com/829417 external/wpt/html/browsers/offline/appcache/workers/appcache-wo
 
 Bug(none) http/tests/misc/redirect-to-about-blank.html [ Timeout ]
 Bug(none) http/tests/security/cors-rfc1918 [ Crash Timeout ]
-Bug(none) http/tests/security/mime-type-execute-as-html-01.html [ Failure ]
-Bug(none) http/tests/security/mime-type-execute-as-html-04.html [ Failure ]
 Bug(none) http/tests/security/powerfulFeatureRestrictions/old-powerful-features-on-insecure-origin.html [ Pass Timeout ]
 
 # Only passes with Network Servce enabled (unfortunately these expectations still