css3/device-adapt/viewport-user-style.html fails when lazy attaching from the parser

This code forces a layout *after* doing the measurement. Force the layout
before grabbing the viewport dimensions.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/23513018

git-svn-id: svn://svn.chromium.org/blink/trunk@157236 bbb929c8-8fbe-4397-9dbb-9b2b20218538