Place a CHECK to investigate an XHR crash

It seems the crash is a null access in a ResourceResponse assignment.
This CL adds a CHECK for that.

Bug: 570946
Change-Id: I8c7afe8f76499f80fe918cd6d6617ff2952a2e6f
Reviewed-on: https://chromium-review.googlesource.com/1123662Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572072}

third_party/blink/renderer/core/xmlhttprequest/xml_http_request.cc

@@ -1757,6 +1757,9 @@ void XMLHttpRequest::DidReceiveResponse(
     unsigned long identifier,
     const ResourceResponse& response,
     std::unique_ptr<WebDataConsumerHandle> handle) {
+  // TODO(yhirano): Remove this CHECK: see https://crbug.com/570946.
+  CHECK(&response);
+
   ALLOW_UNUSED_LOCAL(handle);
   DCHECK(!handle);
   NETWORK_DVLOG(1) << this << " didReceiveResponse(" << identifier << ")";