Make HeapSnapshotLoader understand recent changes to the allocation tracking protocol

BUG=277984
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/177253002

git-svn-id: svn://svn.chromium.org/blink/trunk@168981 bbb929c8-8fbe-4397-9dbb-9b2b20218538