Encoding API: Update tests and simplify constructors

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173754

[jsbell, 2014-05-02 00:16:02]

jshin@ - part 1 of the "replacement" encoding landing.

This is just trivial API test/impl changes, nothing functional. It makes
subsequent patches smaller.

[jsbell, 2014-05-02 18:39:25]

This is part 1 of 3. The full sequence is:

1. API/test cleanup - https://codereview.chromium.org/269593009
2. New codec - https://codereview.chromium.org/265973003
3. Block use by name - https://codereview.chromium.org/261013007

[eseidel, 2014-05-09 06:38:04]

lgtm

[jsbell, 2014-05-09 15:20:05]

The CQ bit was checked by jsbell@chromium.org

[commit-bot: I haz the power, 2014-05-09 15:20:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jsbell@chromium.org/269593009/1

[commit-bot: I haz the power, 2014-05-09 15:31:26]

Change committed as 173754