Adding a tutorial video to the MobileWebApp sample as an example of using HTML5 video. Also fixing issue 6300 (pressing cancel button doesn't work when adding a new task).

Demo at http://jlabanca-testing.appspot.com/.  Click the "Need Help?" link in the upper right corner to view the video.

Issue: 6300

Review at http://gwt-code-reviews.appspot.com/1425815

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10122 8db76d5a-ed1c-0410-87a9-c151d255dfc7