Make sure the --site-per-process flag is present for subframe process swaps.

content/browser/loader/cross_site_resource_handler.cc

Index: content/browser/loader/cross_site_resource_handler.cc
diff --git a/content/browser/loader/cross_site_resource_handler.cc b/content/browser/loader/cross_site_resource_handler.cc
index 44d24ce8a36984fcb8afdb94f50dfe05a1a1613d..ccdd321ef3362af8fe3dcca1c08eb856a004ad93 100644
--- a/content/browser/loader/cross_site_resource_handler.cc
+++ b/content/browser/loader/cross_site_resource_handler.cc
@@ -68,6 +68,13 @@ void OnCrossSiteResponseHelper(const CrossSiteResponseParams& params) {
       RenderFrameHostImpl::FromID(params.global_request_id.child_id,
                                   params.render_frame_id);
   if (rfh) {
+    if (rfh->GetParent()) {
+      // We should only swap processes for subframes in --site-per-process mode.
+      // CrossSiteResourceHandler is not installed on subframe requests in
+      // default Chrome.
+      CHECK(base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kSitePerProcess));
+    }
     rfh->OnCrossSiteResponse(
         params.global_request_id, cross_site_transferring_request.Pass(),
         params.transfer_url_chain, params.referrer,