Add layout test and test media for infinite duration tests.

Adds a test for the corresponding Chrome side change:

http://crrev.com/291504

BUG=400442
TEST=new test!

Review URL: https://codereview.chromium.org/499513002

git-svn-id: svn://svn.chromium.org/blink/trunk@180867 bbb929c8-8fbe-4397-9dbb-9b2b20218538