ES6: String(symbol) should work like symbol.toString

Using String as a function and passing a symbol should return the
same  value as if Symbol.prototype.toString was called.

http://people.mozilla.org/~jorendorff/es6-draft.html#sec-string-constructor-string-value

BUG=v8:3554
LOG=Y
R=rossberg@chromium.org, rossberg

Review URL: https://codereview.chromium.org/564863002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23923 ce2b1a6d-e550-0410-aec6-3dcde31c8c00