Re-enable MSE_ClearKey tests

Re-enable MSE_ClearKey tests

Now that prefixed EME code can add event handlers before calling
generateRequest(), the flakiness problems with these tests is
fixed.

BUG=402766
TEST=re-enabled tests pass

Committed: https://crrev.com/7741919239be94b59370aea90f4dd7ecdf60843b
Cr-Commit-Position: refs/heads/master@{#302328}

[jrummell, 2014-10-31 18:56:33]

jrummell@chromium.org changed reviewers:
+ ddorwin@chromium.org, sandersd@chromium.org, xhwang@chromium.org

[jrummell, 2014-10-31 18:56:34]

PTAL.

[xhwang, 2014-10-31 19:00:28]

lgtm

[jrummell, 2014-10-31 20:30:05]

The CQ bit was checked by jrummell@chromium.org

[commit-bot: I haz the power, 2014-10-31 20:31:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/689153003/1

[commit-bot: I haz the power, 2014-10-31 21:40:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-31 21:41:38]

Patchset 1 (id:??) landed as
https://crrev.com/7741919239be94b59370aea90f4dd7ecdf60843b
Cr-Commit-Position: refs/heads/master@{#302328}