fix update buffer too fast

[mix5003]

when browser updating we can not appendBuffer so we should wait before until updateend before add new chunk.

or may be use queue for speed up like this link?

[ebidel]

Haven't touched this api in a while :)

The demo still works for me: https://html5-demos.appspot.com/static/media-source.html. Is this the cause of the console error?

[mix5003]

in firefox it error in console is InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable

[mix5003]

i just found this patch will fix Failed to execute 'endOfStream' on 'MediaSource': The 'updating' attribute is true on one or more of this MediaSource's SourceBuffers. in
Chrome Version 63.0.3239.108 (Official Build) (64-bit) on Linux Mint 18.3 too

i think it because endOfStream() was call before updateend (it still updating).