MSE: Add basic sequence AppendMode layout tests

Adds some basic layout tests for 'sequence' SourceBuffer.mode.

Without WebM stream parser support, Chromium MP4 parser causes the new
test to fail. Bug 371947 tracks that failure, which will not show on
Chromium bots since they don't support mp4_stream_parser.

R=acolwell@chromium.org
BUG=249422
TEST=(webm on local Linux) http/tests/media/media-source/mediasource-sequencemode-append-buffer.html

Review URL: https://codereview.chromium.org/273153002

git-svn-id: svn://svn.chromium.org/blink/trunk@174336 bbb929c8-8fbe-4397-9dbb-9b2b20218538