Allocation profiler: switch to 1-based positions in the heap snapshots

BUG=None
R=alph@chromium.org

Review URL: https://codereview.chromium.org/27678002

git-svn-id: svn://svn.chromium.org/blink/trunk@159872 bbb929c8-8fbe-4397-9dbb-9b2b20218538