Implement Mirror object for Symbols.

R=rossberg@chromium.org, yurys@chromium.org
BUG=v8:3290
LOG=Y

Review URL: https://codereview.chromium.org/297513006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21414 ce2b1a6d-e550-0410-aec6-3dcde31c8c00