Allocation tracker: use field offsets from snapshot's metainfo

Replaced hardwired values with values evaluated from snapshot's meta node.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/25464003

git-svn-id: svn://svn.chromium.org/blink/trunk@158631 bbb929c8-8fbe-4397-9dbb-9b2b20218538