Fix setting MediaSource's duration
Created by: bbert
When setting duration on MediaSource, if any SourceBuffer is updating it throws an InvalidStateError
exception:
Uncaught DOMException: Failed to set the 'duration' property on 'MediaSource': The 'updating' attribute is true on one or more of this MediaSource's SourceBuffers
This PR fixes this issue by check if no SourceBuffer is updating when setting MediaSource duration