Download FTP resources, do not render them.

With the exception of generated directory listing pages, resources
requested via FTP will be downloaded rather than rendered.

Intent thread: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/eopgOoY1QLs
Spec: https://github.com/whatwg/fetch/pull/839

Bug: 744499
Change-Id: Ic04f17986cbceb511a19d541f4dd4c67685df81a
Reviewed-on: https://chromium-review.googlesource.com/c/1337338
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611125}
8 files changed