Define new class for iteme range returned by heap snapshot worker

BUG=336785
R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/145403004

git-svn-id: svn://svn.chromium.org/blink/trunk@165618 bbb929c8-8fbe-4397-9dbb-9b2b20218538