[OnionSoup] a quick fix for base::internal::flat_tree::insert()

Looks like the operation of base::internal::flat_tree::insert() will
cause a build error in specific version of MSVC's STL.
This CL makes a quick fix for the bug, which still needs a more
generic solution.

The bug is raised here by Martin:
https://crrev.com/c/1312673/9/content/common/service_worker/
service_worker_utils.cc#265

Taiju provides a possible workaround:
https://crrev.com/c/1333171

Change-Id: I55ab4eaecb77ea974b7b651fd32f7982bc79e039
Reviewed-on: https://chromium-review.googlesource.com/c/1331172
Commit-Queue: Richard Li <richard.li@intel.com>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#607547}

content/common/service_worker/service_worker_utils.cc

@@ -261,8 +261,8 @@ ServiceWorkerUtils::DeserializeFetchRequestFromString(
   request_ptr->frame_type = network::mojom::RequestContextFrameType::kNone;
   request_ptr->url = GURL(request_proto.url());
   request_ptr->method = request_proto.method();
-  request_ptr->headers.insert(request_proto.headers().begin(),
-                              request_proto.headers().end());
+  request_ptr->headers = {request_proto.headers().begin(),
+                          request_proto.headers().end()};
   request_ptr->referrer =
       blink::mojom::Referrer::New(GURL(request_proto.referrer().url()),
                                   static_cast<network::mojom::ReferrerPolicy>(