Avoid updating layout in getComputedStyle(e).length

getComputedStyle(e).length should only return 0 if the element is not in
an active document meaning it has no RenderStyle.

BUG=413573

Review URL: https://codereview.chromium.org/582713003

git-svn-id: svn://svn.chromium.org/blink/trunk@182411 bbb929c8-8fbe-4397-9dbb-9b2b20218538