Encoding API: Add TextDecodeOptions and TextDecoderOptions

Replacing generic Dictionary. No changes in behavior.
Tests under fast/encoding cover this change.

BUG=403150

Review URL: https://codereview.chromium.org/564463002

git-svn-id: svn://svn.chromium.org/blink/trunk@181890 bbb929c8-8fbe-4397-9dbb-9b2b20218538