Cancel IPv6 probe jobs during shutdown....

Cancel IPv6 probe jobs during shutdown.
This works around bug where chrome sometimes leaks URLRequests past destruction of the IO thread.

BUG=41986
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=45003

[jar (doing other things), 2010-04-19 18:49:05]

LGTM... but I had a small suggestion to go further below.

http://codereview.chromium.org/1518037/diff/1/2
File net/base/host_resolver_impl.cc (right):

http://codereview.chromium.org/1518037/diff/1/2#newcode854
net/base/host_resolver_impl.cc:854: DiscardIPv6ProbeJob();
This looks good.... but perhaps we could go further, and disable any further
probes.  There is always a chance that the network is being taken down... and
that *might* trigger additional probes... and it would be good to pre-empt such
as well.  

Per your original code-review commentary on my CL, if you set the probe status
explicitly, that will disable the probe.

[eroman, 2010-04-19 21:21:13]

Done. I added a DCHECK that new probe jobs aren't added post Shutdown, and a
short-cuircuit in case they are.

[jar (doing other things), 2010-04-19 21:28:08]

LGTM