Bindings: Add ScalarValueString support

Spec:
http://encoding.spec.whatwg.org/#type-scalarvaluestring

Followup to:
Add ByteString support to IDL bindings
https://codereview.chromium.org/309553002/

Encoding API: Handle null input string
https://codereview.chromium.org/313393005/

BUG=379009
TEST=fast/encoding/api/utf16-surrogates-encode.html
TEST=fast/js/webidl-type-mapping.html

Review URL: https://codereview.chromium.org/313993002

git-svn-id: svn://svn.chromium.org/blink/trunk@176449 bbb929c8-8fbe-4397-9dbb-9b2b20218538