Utilize new MSE changeType() API where available

Created by: wilaw

Work is afoot in the W3C WICG to develop a new changeType method on SourceBuffer that allows the type (bytestream and codec(s)) of media bytes subsequently appended to the SourceBuffer to be changed. This should allow smooth adaption set and period transitions.

https://github.com/wicg/media-source/blob/codec-switching/codec-switching-explainer.md

As of Sept 20 2018, supported on Chrome M70 and Firefox 64.

Request is to develop and implement a poly-fill to allow this new API to be used when available across period and adaption set transitions.