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: svn://svn.chromium.org/blink/trunk@201564 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/TestExpectations

@@ -727,8 +727,6 @@ crbug.com/302729 [ Android ] http/tests/media/media-source/mediasource-seek-beyo
 crbug.com/524160 [ Debug ] http/tests/media/media-source/mediasource-appendbuffer-quota-exceeded.html [ Timeout ]
 crbug.com/524160 [ Debug ] http/tests/media/media-source/mediasource-appendstream-quota-exceeded.html [ Timeout ]
 
-crbug.com/526337 http/tests/media/media-source/mediasource-duration.html [ Pass Failure ]
-
 crbug.com/517449 [ Android ] css3/images/optimize-contrast-image.html [ ImageOnlyFailure ]
 
 crbug.com/309675 compositing/gestures/gesture-tapHighlight-simple-longPress.html [ ImageOnlyFailure ]