Encoding API: Update tests and simplify constructors

Add a snapshot of the WHATWG Encoding spec's encodings.js and document
the deltas from it in shared.js. Skip testing "replacement" encodings.

Also, Simplify the constructors by just passing through a TextEncoding
object rather than digging out a string then recreating it.

No functional changes.

BUG=277037
R=jshin@chromium.org

Review URL: https://codereview.chromium.org/269593009

git-svn-id: svn://svn.chromium.org/blink/trunk@173754 bbb929c8-8fbe-4397-9dbb-9b2b20218538