Allow buffer removal when MediaSource is ended

[epiclabsDASH]

Fix #2415.

In some situations, like when there are track changes, we have to allow the buffer to be removed even when it has reached the end of the stream. This PR adds the possibility of indicating to SourceBufferController that a buffer must be removed, independently of the playback context, and use it to force removal of buffers when there are track switches.

[nicosang]

according to the spec...

If the readyState attribute of the parent media source is in the "ended" state then run the following steps:

Set the readyState attribute of the parent media source to "open"
Queue a task to fire a simple event named sourceopen at the parent media source.

My question is : is there some troubles with this sourceopen event?

[epiclabsDASH]

Good point. Currently that doesn't have any effect because the listener for sourceopen event is removed as soon as the first sourceopen event is received.