Update mediasource-duration-expected to PASS and mark flaky temporarily

Update mediasource-duration-expected to PASS and mark flaky temporarily

Chromium-side patch that removes suppressing asynch processing of
redundant MSE seeks (https://codereview.chromium.org/1305273007/)
appears to have changed behavior enough on at least desktop Chromium
that the mediasource-duration layout test now shows "PASS" for each test
within it. This change updates those expectations to PASS (by removing
the expectations file) and marks the test flaky in preparation
for landing the Chromium-side change.

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=201511

[wolenetz, 2015-08-31 23:47:07]

The CQ bit was checked by wolenetz@chromium.org

[commit-bot: I haz the power, 2015-08-31 23:47:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1321253005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1321253005/1

[commit-bot: I haz the power, 2015-09-01 00:32:55]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201511