MSE: Fix flaky mediasource-append-buffer.html test

Media source specification's end of stream algorithm is currently
underspecified when no media data is buffered, and implementation may
transition beyond 'ended' to 'closed' MediaSource readyState.

This change updates the layout test to cover the well-specified case
when there is media data buffered prior to endOfStream() and removes the
flaky fail expectation. Later tests will be added to cover the currently
underspecified cases, once correct behavior is defined and implemented.

BUG=399188
R=acolwell@chromium.org
TEST=mediasource-append-buffer.html passed 100/100 attempts with 8 content_shells in parallel locally on linux

Review URL: https://codereview.chromium.org/417163006

git-svn-id: svn://svn.chromium.org/blink/trunk@179357 bbb929c8-8fbe-4397-9dbb-9b2b20218538