Do not sniff mime type for fetch() requests
1. The mime sniffer buffers some body data. This is a very confusing
behavior for streaming usecases.
2. The fetch spec uses the MIME type with parameters which
WebURLResponse::mimeType doesn't provide. We need to parse
Content-Type header in any case and mime sniffing may introduce
inconsistency here.
3. We don't see any benefit of mime sniffing for fetch() initiated
requests.
From the above reasons we don't want the mime sniffer to run for fetch()
initiated requests.
BUG=2016
Review-Url: https://codereview.chromium.org/2323623002
Cr-Commit-Position: refs/heads/master@{#419994}
6 files changed
