https://people.mozilla.org/~jorendorff/es6-draft.html#sec-tonumber

Based on patch from caitp <caitpotter88@gmail.com>
https://codereview.chromium.org/454233002/

BUG=v8:3499
LOG=Y
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/458753004

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23057 ce2b1a6d-e550-0410-aec6-3dcde31c8c00