[MSE] Blink MSE tests submitted to web-platform-test project

FYI. I've submitted a pull
request<https://github.com/w3c/web-platform-tests/pull/725> for
the Blink MSE tests. These test all pass on Chrome with WebM and MP4. I
expect there will be differences between UA implementations with any of the
tests that include media timestamps. We are still working on bringing
Chrome in line with what the spec actually says. These should be a good
starting point for establishing baseline interop though.

Aaron