Improve unprefixed MediaSource test coverage.

These changes add test cases for the unprefixed MediaSource API that are
derived from the existing prefixed MediaSource tests. These are being
added so that we don't lose test coverage when the prefixed API and
tests are removed.

BUG=363444

Review URL: https://codereview.chromium.org/245893002

git-svn-id: svn://svn.chromium.org/blink/trunk@172087 bbb929c8-8fbe-4397-9dbb-9b2b20218538