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

Review URL: https://codereview.chromium.org/1325823005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@201564 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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