Count ArrayBuffer's backing_store memory in heap snapshot.

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

Review URL: https://codereview.chromium.org/163593002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19356 ce2b1a6d-e550-0410-aec6-3dcde31c8c00