Adding HTML5 track(caption test) case in PyAuto media functional tests.

The track specification can be found in 
http://www.w3.org/TR/html5/video.html#the-track-element. This is the first
version of the tests so there is no verification yet. -g option is used in the media_test_runner to specify the track(caption) file location. If it is not set, default track file is used (the same location as video and file name is 'title_en.vtt'.


Review URL: http://codereview.chromium.org/7003131

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88940 0039d316-1c4b-4281-b951-d872f2087c98