Make "three snapshot technique" work on heap snapshots loaded from files

This changes allows to show objects created between two selected heap snapshots in the case when the snapshots have been loaded from files.

BUG=154084
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/23609064

git-svn-id: svn://svn.chromium.org/blink/trunk@158273 bbb929c8-8fbe-4397-9dbb-9b2b20218538