Fix flaky cast_unittests where threads were being joined incorrectly.

Fix flaky cast_unittests where threads were being joined incorrectly.

Root cause of the flakiness is that the StandaloneCastEnvironment, as a
ref-counted object, was being destroyed on a thread other than the one
that created it.  At destruction, it attempted to join on the threads it
owns, but these threads must be joined with the thread that they were
spawned from.

BUG=396480
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=287194

[miu, 2014-07-31 22:56:02]

hubbe: PTAL.

[miu, 2014-08-01 20:16:42]

hclam: PTAL.

[Alpha Left Google, 2014-08-01 20:18:05]

lgtm

[miu, 2014-08-01 20:22:19]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-08-01 20:24:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/miu@chromium.org/436673004/1

[commit-bot: I haz the power, 2014-08-02 11:19:46]

Change committed as 287194