Fix video progress slider method.

In each Video JavaScript module, methods are either private,
or public. When a method is public, it is made available via
the state variable. It seems that a while ago one method
was made public, but throughout the code it is used as a private
method. This has been corrected.
Merge pull request #2273 from edx/valera/fix_video_progress_slider_method

Fix video progress slider method.