instanceof HTMLMediaElement failfast messes up with MSE polyfill

Created by: AxelDelmas

Is it actually necessary to run this check ?

It fails if we pass a polyfill. We used to work around this by making our polyfill inherit from HTMLMediaElement, but this change causes an exception, as some checks are run in HTMLMediaElement's src setter.

See corresponding fmse issue here: https://github.com/streamroot/fmse/issues/9