DevTools: fix erroneous box size reported in metrics panel

This patch fixes "NaN x NaN" box size reported for metrics panel in certain cases.

BUG=282128

Review URL: https://chromiumcodereview.appspot.com/23799003

git-svn-id: svn://svn.chromium.org/blink/trunk@156994 bbb929c8-8fbe-4397-9dbb-9b2b20218538