DevTools: Refactor the way child tree elements are updated for nodes on element panel.

This is done in preparation for distributed nodes inspection support.
R=apavlov, pfeldman

Review URL: https://codereview.chromium.org/702133002

git-svn-id: svn://svn.chromium.org/blink/trunk@184970 bbb929c8-8fbe-4397-9dbb-9b2b20218538