Add [TreatReturnedNullStringAs] support for ByteString.

If my understanding is correct, we need "[TreatReturnedNullStringAs=Null] ByteString get(ByteString key);" for Headers class to return null.
http://fetch.spec.whatwg.org/#dom-headers-get


BUG=347426

Review URL: https://codereview.chromium.org/337373002

git-svn-id: svn://svn.chromium.org/blink/trunk@176370 bbb929c8-8fbe-4397-9dbb-9b2b20218538