On the about page, only serve Opera mp4. Pre-Chromium Opera and MediaElement.js doesn't load ogv (video/ogg). see #24832.

Merges [24950] to the 3.6 branch.



git-svn-id: https://core.svn.wordpress.org/branches/3.6@24951 1a063a9b-81f0-0310-95a4-ce76da25c4cd