Introduce interface for heap snapshot grid node children provider

BUG=342303
R=alph@chromium.org, apavlov@chromium.org

Review URL: https://codereview.chromium.org/176973008

git-svn-id: svn://svn.chromium.org/blink/trunk@168651 bbb929c8-8fbe-4397-9dbb-9b2b20218538