Block 'ftp:' subresource requests from non-'ftp:' pages.

Usage of the 'ftp:' protocol when requesting subresources from non-'ftp:'
clients has slowly declined over the last few years to the point where
it represents a [negligable amount of traffic][1]. The protocol does not
support modern requirements, like encryption, and we're interested in
removing support from //net.

To that end, this patch alters Fetch to block FTP subresources from
webby clients. That is, a page delivered from `http://example.com/`
will receive a network error response to requests like those generated
from `<img src='ftp://example.com/image.png'>`.

Intent: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/bIJdwwoQ98U/-F1aL2FgBAAJ
PR against Fetch: https://github.com/whatwg/fetch/pull/464.

BUG=435547

Review-Url: https://codereview.chromium.org/2656443005
Cr-Commit-Position: refs/heads/master@{#451709}
4 files changed