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This aligns behavior more closely FireFox, though less closely
with Edge and Safari.  See RFC at https://mimesniff.spec.whatwg.org/
(This behavior is also more consistent with
https://tools.ietf.org/html/rfc7231#section-3.1.1.1)

In particular, no longer split the string around semi-colons first,
but instead parse character-by-character (Which only really affects
weird things like charset=";"), and use backslash as an escape
character in quoted string values.  This affects both charset and
boundary parameters, so we should keep an eye out for bug reports
about either of those breaking in corner cases.

This also applies the quoted string parsing logic to Content-Type
boundary parameters (It was just used for charsets before) - this is
unlikely to break anything, since the one consumer of that field
arbitrarily removes all leading/trailing spaces and quotes from
boundary strings.

Bug: 774429
Change-Id: I28c9c035e1c1c28e181bd0e8005266cfd63af7e9
Reviewed-on: https://chromium-review.googlesource.com/990995
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548008}