Encoding API: Expose 'fatal' and 'ignoreBOM' readonly attributes

From private conversation with annevk - make the properties of the
TextDecoder object inspectable.

BUG=375847

Review URL: https://codereview.chromium.org/292243003

git-svn-id: svn://svn.chromium.org/blink/trunk@174977 bbb929c8-8fbe-4397-9dbb-9b2b20218538