Add minimal redundant media source seek layout test

Adds media source layout test to verify the Chrome for Android redundant seek suppression optimization fix in r226166 works as expected.

BUG=267111
R=acolwell@chromium.org
TEST=New layout test passes on ToT desktop, Chrome for Android, and fails with timeout on Chrome for Android with r226166 reverted (expected due to ChunkDemuxer pending StartWaitingForSeek+seek to ephemeral time that was optimized away prior to r226166).

Review URL: https://codereview.chromium.org/25560004

git-svn-id: svn://svn.chromium.org/blink/trunk@158676 bbb929c8-8fbe-4397-9dbb-9b2b20218538