Don't sniff HTML from documents delivered via the file protocol

To reduce attack surface, Chrome should not MIME-sniff to text/html for
any document delivered via the file protocol. This change only impacts
the file protocol (documents served via HTTP/HTTPS/etc are unaffected).

Bug: 777737
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I7086454356b8d2d092be9e1bca0f5ff6dd3b62c0
Reviewed-on: https://chromium-review.googlesource.com/751402
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Lawrence <elawrence@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514372}
5 files changed