Audio and Video cleanup.

Add a MediaBase widget backing Audio and Video, and clean up tests. Also,
address comments by reviewer and re-encode test media files for a smaller size
and to fix an h264 video issue.

Review at http://gwt-code-reviews.appspot.com/1415801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10001 8db76d5a-ed1c-0410-87a9-c151d255dfc7