Add some media source extension layout tests to SmokeTests

BUG=315949
TEST=These tests pass on Nexus 4 and are pretty fast.

Review URL: https://codereview.chromium.org/115463003

git-svn-id: svn://svn.chromium.org/blink/trunk@163978 bbb929c8-8fbe-4397-9dbb-9b2b20218538