Do not set utf-8 encoding in OfflinePageURLLoader

Setting it will skip the auto detection of response encoding.

Bug: 966094
Change-Id: I601bbc99339a94ec314f52af4d38236cdd457017
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1650086Reviewed-by: Dmitry Titov <dimich@chromium.org>
Commit-Queue: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667353}

chrome/browser/offline_pages/offline_page_url_loader.cc

@@ -293,7 +293,6 @@ void OfflinePageURLLoader::OnReceiveResponse(
   }
 
   response_head.mime_type = "multipart/related";
-  response_head.charset = "utf-8";
   response_head.content_length = file_size;
 
   client_->OnReceiveResponse(response_head);