Update ObjectToString to Harmony-draft algorithm

Updates Object.prototype.toString() to use algorithm described in harmony drafts.

Currently, the behaviour is essentially the same as ES262's version, however this changes when internal structures
such as Promise make use of symbolToStringTag (as they are supposed to, see v8:3241), and changes further once
Symbol.toStringTag is exposed publicly.

BUG=v8:3241, v8:3502
LOG=N
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/546803003

Patch from Caitlin Potter <caitpotter88@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24783 ce2b1a6d-e550-0410-aec6-3dcde31c8c00