Fix some shutdown problems in tab capture performance tests.

Fix some shutdown problems in tab capture performance tests.

As usual, the RunUntilIdle() call causes problems. Calling stop() on the
webrtc stream apparently doesn't propagate back to the capture, so make sure
that we call stop() on the capture stream explicitly.

Also, raise frame rate to 60 fps.

Not entirely sure if this will fix the attached bug or not, but either way it's a problem that should be fixed.

BUG=474808
Committed: https://crrev.com/f8c0510631f52de6ca585fdce1ac171b37f36244
Cr-Commit-Position: refs/heads/master@{#324325}

[hubbe, 2015-04-08 19:07:59]

hubbe@chromium.org changed reviewers:
+ miu@chromium.org

[miu, 2015-04-08 20:25:46]

The CQ bit was checked by miu@chromium.org

[miu, 2015-04-08 20:25:46]

lgtm

[commit-bot: I haz the power, 2015-04-08 20:27:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1076533002/20001

[commit-bot: I haz the power, 2015-04-09 01:02:54]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-09 01:04:03]

Patchset 2 (id:??) landed as
https://crrev.com/f8c0510631f52de6ca585fdce1ac171b37f36244
Cr-Commit-Position: refs/heads/master@{#324325}