Fix race between ~SkThreadPool and SkThreadPool::Loop on fDone.

Fix race between ~SkThreadPool and SkThreadPool::Loop on fDone.

We're writing fDone without holding the mutex.  Bad form, says tsan.

In practice this is fairly innocuous, as fDone only ever goes from false to
true and only once.  Though, I wouldn't be surprised if there were some way
this could leak a thread that never got the signal to die.

BUG=

Committed: http://code.google.com/p/skia/source/detail?r=11563

[scroggo, 2013-10-01 15:21:21]

lgtm

[commit-bot: I haz the power, 2013-10-01 15:24:53]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/mtklein@google.com/25371003/1

[commit-bot: I haz the power, 2013-10-01 15:24:56]

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[mtklein, 2013-10-01 15:25:42]

Mike, can you approve my indentation?

[reed1, 2013-10-01 15:27:02]

lgtm

[commit-bot: I haz the power, 2013-10-01 15:27:43]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/mtklein@google.com/25371003/1

[commit-bot: I haz the power, 2013-10-01 18:44:36]

Change committed as 11563