DevTools: Fix style UI in object preview in console.

In console preview "null" should have same style as "undefined".

R=yurys, vsevik

Review URL: https://codereview.chromium.org/517523002

git-svn-id: svn://svn.chromium.org/blink/trunk@181039 bbb929c8-8fbe-4397-9dbb-9b2b20218538