MSE: Marks mediasource-addsourcebuffer test slow on Win7 Release

MSE: Marks mediasource-addsourcebuffer test slow on Win7 Release

Moves this test from FlakyTests TIMEOUT expectation to SlowTests.
This test frequently takes over 6 seconds, and on Win7 Release bots
results in occasional timeouts.

BUG=24182
R=philipj@opera.com

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196385

[wolenetz, 2015-06-03 02:47:47]

The CQ bit was checked by wolenetz@chromium.org to run a CQ dry run

[wolenetz, 2015-06-03 02:48:00]

Please take a look at patch set 1.
Thanks!
Matt

[commit-bot: I haz the power, 2015-06-03 02:48:34]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1161393006/1

[commit-bot: I haz the power, 2015-06-03 04:01:36]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-03 04:01:38]

Dry run: This issue passed the CQ dry run.

[philipj_slow, 2015-06-03 09:19:30]

The CQ bit was checked by philipj@opera.com

[philipj_slow, 2015-06-03 09:19:31]

lgtm

[commit-bot: I haz the power, 2015-06-03 09:19:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1161393006/1

[commit-bot: I haz the power, 2015-06-03 09:23:49]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196385