Update harmony Object.prototype.toString to 2/2/2015 spec

- Removes special handling of non-string @@toStringTag values (use builtinTag)
- Removes special handling of @@toStringTags which match [[Class]] names (remove ~ prefix)

BUG=v8:3502
R=arv@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/895633004

Cr-Commit-Position: refs/heads/master@{#26411}