Re-enable PrefixedVideoFullscreen

This feature is deprecated and we'd like to remove it, but there are still some
web sites that are using it. This CL re-enables the feature while we evangalize
those sites.

BUG=346236

Review URL: https://codereview.chromium.org/240373006

git-svn-id: svn://svn.chromium.org/blink/trunk@171877 bbb929c8-8fbe-4397-9dbb-9b2b20218538