Add tests to verify playback ends when audio/video durations differ.

Add tests to verify playback ends when audio/video durations differ.

In preparation for changing how we compute media time, add some tests to
verify the current behaviour.

BUG=370634
Committed: https://crrev.com/d6f1e09b967d780c72d68fc1869a50ffdf74bd66
Cr-Commit-Position: refs/heads/master@{#292544}

[scherkus (not reviewing), 2014-08-29 00:38:26]

scherkus@chromium.org changed reviewers:
+ dalecurtis@chromium.org

[scherkus (not reviewing), 2014-08-29 00:38:26]

I couldn't find any existing tests for this scenario ... but it is indeed an
edge case where we've had issues before.

[DaleCurtis, 2014-08-29 00:52:10]

lgtm, we have some pipeline tests around different start times, which has some
implicit testing like this, but it's good for something more explicit.

[scherkus (not reviewing), 2014-08-29 00:53:19]

The CQ bit was checked by scherkus@chromium.org

[commit-bot: I haz the power, 2014-08-29 00:54:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/scherkus@chromium.org/517843003/1

[commit-bot: I haz the power, 2014-08-29 01:50:48]

Committed patchset #1 (id:1) as 505fb8a927fe5ff73c7541f10a28214a854b8a11

[commit-bot: I haz the power, 2014-09-10 03:05:07]

Patchset 1 (id:??) landed as
https://crrev.com/d6f1e09b967d780c72d68fc1869a50ffdf74bd66
Cr-Commit-Position: refs/heads/master@{#292544}