DevTools: [Console] noop instead of DOM rebuilding in certain cases.

This patch avoids DOM rebuilding if the set of viewported elements does
not change.

BUG=385029

Review URL: https://codereview.chromium.org/377553002

git-svn-id: svn://svn.chromium.org/blink/trunk@177656 bbb929c8-8fbe-4397-9dbb-9b2b20218538