DOMException use String instead of char*

Second try, this time with fix to not have different types in ?:

BUG=None

Review URL: https://chromiumcodereview.appspot.com/19549006

git-svn-id: svn://svn.chromium.org/blink/trunk@154558 bbb929c8-8fbe-4397-9dbb-9b2b20218538