Switch the NSS thread from being a base::Thread to a base::SequencedWorkerPool of 1

Switch the NSS thread from being a base::Thread to a base::SequencedWorkerPool of 1

BUG=135435
TEST=existing + tsan bots


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=145977

[Ryan Hamilton, 2012-07-10 17:12:35]

lgtm

Is the motivation here that SequencedWorkerPool are the new hotness and should
be used?

[Ryan Sleevi, 2012-07-10 17:26:40]

On 2012/07/10 17:12:35, Ryan Hamilton wrote:
> lgtm
> 
> Is the motivation here that SequencedWorkerPool are the new hotness and should
> be used?

Yes and no...

As mentioned on the bug, base::Thread/base::MessageLoop (which base::Thread
provides) "shouldn't" be used on an unjoined worker thread. This is because they
may access Singletons (such as MessageLoop::current() or base::Time::Now()).
This is because the Singletons are torn down by the main thread's AtExitHandler,
so during shutdown, the worker thread might try to access something that had
already been freed.

Up until recently, they didn't, so the code worked fine and without any races.
Then a change in base/ caused MessageLoop::current() to be called by
MessageLoop::Run(), and that caused shutdown races. Rather than making the
Singleton/LazyInstances Leaky, I switched this to use the SequencedWorkerPool,
which is designed to be abandonable / have unjoined worker threads.

The "Yes" part is that SequencedTaskRunners are the new hotness over MessageLoop
for "Runs a series of tasks". That is, a thread can have a SequencedTaskRunner,
but not have a MessageLoop::current(). This is useful for unjoined worker
threads (which cannot have a MessageLoop). Since we don't (yet) have any "Create
a single unjoined worker thread with a TaskRunner" facility, I use a worker pool
with a thread-cap of 1 to simulate it.

[commit-bot: I haz the power, 2012-07-10 22:04:20]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rsleevi@chromium.org/10749009/11001

[commit-bot: I haz the power, 2012-07-10 22:56:24]

Change committed as 145977