Fix flakyness in PrefMemberTest::MoveToThread

Fix flakyness in PrefMemberTest::MoveToThread

 In MoveToThread, a thread object is owned by a ref-counted object that
is passed around thread. It means the thread object can be deleted on
a thread that is not the one that created it, which will trigger a
DCHECK. This CL ensures that the second thread is always stopped on the main
thread.

R=battre@chromium.org
BUG=None


Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=174390

[battre, 2012-12-21 10:23:15]

lgtm

[commit-bot: I haz the power, 2012-12-21 10:31:56]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/qsr@chromium.org/11624023/1

[commit-bot: I haz the power, 2012-12-21 13:14:33]

Retried try job too often on linux_chromeos for step(s) browser_tests

[commit-bot: I haz the power, 2012-12-21 13:15:22]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/qsr@chromium.org/11624023/1

[commit-bot: I haz the power, 2012-12-21 14:08:06]

Change committed as 174390