Expose the content of Maps and WeakMaps through MapMirror.

BUG=v8:3291
LOG=N
R=aandrey@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/398513005

Patch from Alexandra Mikhaylova <amikhaylova@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22452 ce2b1a6d-e550-0410-aec6-3dcde31c8c00