Avoid race in RampUpTest

Avoid race in RampUpTest

Poller thread is currently started in the constructor, so the first call
to PollStats() may happen even before the streams have been configured.
This will blow up on RTC_DCHECK_GT(expected_bitrate_bps_, 0);

Thread should instead be started on PerformTest() call.

BUG=

Committed: https://crrev.com/a050e982b02b2faf59202a71b86a70328f372e97
Cr-Commit-Position: refs/heads/master@{#10149}

[sprang_webrtc, 2015-10-02 11:33:55]

sprang@webrtc.org changed reviewers:
+ stefan@webrtc.org

[stefan-webrtc, 2015-10-02 12:57:17]

lgtm, thanks for fixing

[sprang_webrtc, 2015-10-02 12:57:51]

The CQ bit was checked by sprang@webrtc.org

[commit-bot: I haz the power, 2015-10-02 12:57:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1378303004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1378303004/1

[commit-bot: I haz the power, 2015-10-02 13:51:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-02 13:52:00]

Patchset 1 (id:??) landed as
https://crrev.com/a050e982b02b2faf59202a71b86a70328f372e97
Cr-Commit-Position: refs/heads/master@{#10149}