Fix for BLD-708: Firefox YouTube video start time.

In Firefox YouTube video with start time plays from 00:00:00.
The reason was that seekTo() could not start playing a video from
a point that hasn't been buffered yet. Why this started happening
just now, really is hard to explain.

BLD-708.
Merge pull request #2316 from edx/valera/fix_firefox_youtube_start_time

Fix for BLD-708: Firefox YouTube video start time.