Support ES6 weak collections in heap profiler

BUG=chromium:376196
LOG=Y
R=alph@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/294163005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00