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

(Attempt #2, after a speculative revert from a flaky test.)

BUG=461494
TEST=Crash report should show that --site-per-process flag is present.

Review URL: https://codereview.chromium.org/972773003

Cr-Commit-Position: refs/heads/master@{#318798}

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,