Windows: Allow customization of background color (issue #1161).

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk/cef3@1559 5089003a-bbd8-11dd-ad1f-f1f9622dbc98
Mac: Allow customization of background color (issue #1161)

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk/cef3@1560 5089003a-bbd8-11dd-ad1f-f1f9622dbc98