Expose promise value through promise mirror.

R=rossberg@chromium.org, yurys@chromium.org
BUG=v8:3093
LOG=Y

Review URL: https://codereview.chromium.org/258823012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21003 ce2b1a6d-e550-0410-aec6-3dcde31c8c00