Add a UseCounter for <body bgproperties=fixed>

This is a survivor of the first browser wars, implemented by Internet
Explorer but not Netscape. It was never standardized, so measure usage
to figure out the best path forward.

For the archaeologist:
http://lists.w3.org/Archives/Public/www-talk/1996JulAug/0177.html
http://lists.w3.org/Archives/Public/www-talk/1997SepOct/0008.html
http://lists.w3.org/Archives/Public/www-html/1998Oct/0016.html
https://bugzilla.mozilla.org/show_bug.cgi?id=7474
https://projects.kde.org/projects/kde/kdelibs/repository/revisions/34867245

Review URL: https://codereview.chromium.org/428363002

git-svn-id: svn://svn.chromium.org/blink/trunk@179314 bbb929c8-8fbe-4397-9dbb-9b2b20218538