DevTools: cache child node count of bound nodes in DOM agent.

BUG=352497
R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/196353017

git-svn-id: svn://svn.chromium.org/blink/trunk@169227 bbb929c8-8fbe-4397-9dbb-9b2b20218538