Import MSE tests from web-platform-tests

Some of the upstream tests originate from http/tests/media/media-source/
and in the long run these should be consolidated. Importing is the first
step, and may reveal interop problems in need of attention.

BUG=624037,633669,651765
R=chcunningham@chromium.org,wolenetz@chromium.org

Review-Url: https://codereview.chromium.org/2830203003
Cr-Commit-Position: refs/heads/master@{#466370}
1 file changed