Adding a few new MSE tests based on the spec.

This is intended to make the MSE implementation more spec compliant.
More such CLs to follow after this initial round is checked in.

BUG=
TEST=new test pass

Review URL: https://codereview.chromium.org/267333006

git-svn-id: svn://svn.chromium.org/blink/trunk@173667 bbb929c8-8fbe-4397-9dbb-9b2b20218538