Implement step 6 of media `time marches on` algorithm. Improves stability of media WPTs
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes
This should help with the WPTs problems observed in https://github.com/servo/servo/pull/22348#issuecomment-446369831
Unfortunately, GStreamer does not seem to be very reliable with Ogg (check https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/520) and most of the media-element WPTs uses a short ogv file, so I had to make our `canPlayType` report that it is not able to play this type to make the tests pick the alternative mp4 version of the same files. Once Ogg support for GStreamer improves, we should be able to revert this change.
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