fix: replay case

[xiaoyuhen]

fix: replay case

Description

the media source readState is ended when we click the replay button, so we cannot append new buffer to the media source.

The solution is we append an empty buffer to source buffer before we try click replay button or move the slider. Then the media source readState will reopen.
see https://github.com/servo/servo/blob/master/tests/wpt/web-platform-tests/media-source/mediasource-append-buffer.html#L144

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules