Implement "replacement" text encoding.

The WHATWG Encoding Standard redefines certain legacy encodings
as that are only used as XSS attack vectors as aliases for an
abstract "replacement" encoding.

* Decode (of resources) terminates with a single U+FFFD.
* Encode (e.g. for form submissions) is treated as UTF-8.

This is dependent on http://crrev.com/269593009

BUG=277037
R=jshin@chromium.org,eseidel@chromium.org

Review URL: https://codereview.chromium.org/265973003

git-svn-id: svn://svn.chromium.org/blink/trunk@173954 bbb929c8-8fbe-4397-9dbb-9b2b20218538