Replace responseType mismatch handling code in XMLHttpRequest::responseJSONSource() with ASSERT

Like responseBlob() and responseArrayBuffer() method, this method is called
from V8XMLHttpRequestCustom where it's responseType is guaranteed to be
ResponseTypeJSON. So, just ASSERT it.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23533007

git-svn-id: svn://svn.chromium.org/blink/trunk@156909 bbb929c8-8fbe-4397-9dbb-9b2b20218538