mdb_v8, a post-mortem debugging tool for Node.js, allows users to
inspect ScopeInfo structures in order to get more information about
closures.

Currently, it hardcodes the metadata it uses to find this information.
This change allows it to get this metadata from the node binary itself,
and thus to adapt to future changes made to the layout of the ScopeInfo
data structure.

BUG=

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1350843003

Cr-Commit-Position: refs/heads/master@{#30843}