Place CHECKs to investigate an XHR crash

Bug: 570946
Change-Id: Iba0dd0c5af2d44a8baff9c4e5764fb9a624f719c
Reviewed-on: https://chromium-review.googlesource.com/1056929Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558204}

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

@@ -1119,7 +1119,8 @@ void XMLHttpRequest::CreateRequest(scoped_refptr<EncodedFormData> http_body,
     if (upload_)
       request.SetReportUploadProgress(true);
 
-    DCHECK(!loader_);
+    // TODO(yhirano): Turn this CHECK into DCHECK: see https://crbug.com/570946.
+    CHECK(!loader_);
     DCHECK(send_flag_);
     loader_ = ThreadableLoader::Create(execution_context, this, options,
                                        resource_loader_options);
@@ -1182,6 +1183,8 @@ void XMLHttpRequest::Dispose() {
   probe::detachClientRequest(GetExecutionContext(), this);
   progress_event_throttle_->Stop();
   InternalAbort();
+  // TODO(yhirano): Remove this CHECK: see https://crbug.com/570946.
+  CHECK(!loader_);
 }
 
 void XMLHttpRequest::ClearVariablesForLoading() {