Force background color to white to work on Chromecast.

This is live at https://cool-stip-dot-sheriff-o-matic.appspot.com/

NOTRY=true

Review URL: https://codereview.chromium.org/681033002

git-svn-id: svn://svn.chromium.org/blink/trunk@184478 bbb929c8-8fbe-4397-9dbb-9b2b20218538