Fix media layout tests that made invalid assumptions about timeupdate.

Blink r182039 removed unnecessary checks against firing timeupdate
events in response to ece894570c5f32f7f404f4c8da9ff8b9f03b75cd, which
made it possible for the currentTime value to remain the same for
brief periods of time.

The media controller tests were accidentally relying on the timeupdate
event that gets automatically fired when MediaController issues a seek.
Instead, we should wait for that seek to complete before issuing play()
and waiting for the timeupdate event that follows.

The audio-concurrent-supported.html test picked a magic number of
timeupdate events (two, to be exact) before inspecting whether
currentTime increased past zero. Again, because we now fire non-periodic
timeupdate events reliably, this test is picking up on the wrong events
to make it's judgement on what currentTime should be.

BUG=414649

Review URL: https://codereview.chromium.org/574883002

git-svn-id: svn://svn.chromium.org/blink/trunk@182122 bbb929c8-8fbe-4397-9dbb-9b2b20218538