Remove flaky expectation for mediasource-duration test

Remove flaky expectation for mediasource-duration test

The chromium-side fix in https://crrev.com/a609797f appears to have
fixed the flakiness of this test, per the dashboard. This change removes
the flakiness expectation so we can now catch regressions impacting this
test in the future.

TBR=philipj@opera.com
BUG=526337, 399523, 303419, 284782, 266631
TEST=http/tests/media/media-source/mediasource-duration.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201564

[wolenetz, 2015-09-01 16:14:51]

I'll send to CQ shortly.

[wolenetz, 2015-09-01 16:14:55]

The CQ bit was checked by wolenetz@chromium.org

[commit-bot: I haz the power, 2015-09-01 16:15:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1325823005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1325823005/1

[commit-bot: I haz the power, 2015-09-01 17:41:12]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201564

[philipj_slow, 2015-09-02 08:35:44]

lgtm