Simplify media/audio-concurrent-supported.html thanks to r182442.

The first periodic timeupdate event now reliably fires only when
currentTime is non-zero. No need to do this silly "count the number
of timeupdate events and make assumptions about how things work"
dance.

Review URL: https://codereview.chromium.org/600513002

git-svn-id: svn://svn.chromium.org/blink/trunk@182577 bbb929c8-8fbe-4397-9dbb-9b2b20218538