Add support for Chrome-based browsers

Chrome-based browsers don't appear to respect the presentation time of media files added to SourceBuffer, although they work fine with Firefox.

We currently pass `-ss ... -t ... -i ... -movflags frag_keyframe+delay_moov+default_base_moof` to ffmpeg.

Start by tracing the SourceBuffer media path and the ffmpeg invocation that uses `-ss`, `-t`, `-i`, and the listed movflags. Compare the resulting presentation-time behavior in Chrome-based browsers with Firefox, then verify that media duration and playback timing are respected in both.

Written by the indexing model from the issue text.