It turned out that JSArrayBuffer's may share their backing_store so
the backing_store should go through hash map registration just like
other heap objects, so they won't be reported twice.

BUG=341741
LOG=N
R=dslomov@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/166993002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19416 ce2b1a6d-e550-0410-aec6-3dcde31c8c00