Add an opt-out header for HTTP throttling. Never throttle for localhost.

net/url_request/url_request_http_job.cc

Index: net/url_request/url_request_http_job.cc
diff --git a/net/url_request/url_request_http_job.cc b/net/url_request/url_request_http_job.cc
index dd6d63e10314206312b4e219a6a3555e6935921a..400d47e8a9909aa906c19cf57b60d9dfef3eb601 100644
--- a/net/url_request/url_request_http_job.cc
+++ b/net/url_request/url_request_http_job.cc
@@ -210,7 +210,8 @@ void URLRequestHttpJob::NotifyHeadersComplete() {
   if (!is_cached_content_) {
     URLRequestThrottlerHeaderAdapter response_adapter(
         response_info_->headers);
-    throttling_entry_->UpdateWithResponse(&response_adapter);
+    throttling_entry_->UpdateWithResponse(request_info_.url.host(),
+                                          &response_adapter);
   }
 
   ProcessStrictTransportSecurityHeader();