Fix flaky test #12

"CC button works correctly w/o english transcript in HTML5 mode of Video component"
Fix lms video #20: scroll view to be clickable.
Merge pull request #3198 from edx/alex/fix_flaky_video_test_12

Fix flaky test #12
Merge pull request #3199 from edx/alex/fix_flaky_video_test_20

Alex/fix flaky video test 20
Refactor video caption module.