Fixed flakyness of PolicyTest.FileURLBlacklist.

Fixed flakyness of PolicyTest.FileURLBlacklist.

The problem was introduced in https://codereview.chromium.org/110643005.
The URLBlacklistManager used to build the blacklist in the FILE thread
but it's now using the blocking pool; however, FlushBlacklistPolicy
wasn't updated to flush the blocking pool instead of the FILE thread.

This test (and the URL blacklist tests too) have been flaking now
and then on every bot but this race is extremely hard to trigger.
DrMemory on Windows was a consistent repro though.

BUG=339240
Committed: https://crrev.com/8d2d9254a094cb485874336117c8ca7960296c3d
Cr-Commit-Position: refs/heads/master@{#292874}

[Joao da Silva, 2014-08-29 14:13:58]

joaodasilva@chromium.org changed reviewers:
+ pastarmovj@chromium.org

[Joao da Silva, 2014-08-29 14:13:59]

Julian, PTAL.

Daniel, Jeffrey FYI

Thanks!

[pastarmovj, 2014-09-01 14:15:13]

lgtm

[Joao da Silva, 2014-09-01 14:17:33]

The CQ bit was checked by joaodasilva@chromium.org

[commit-bot: I haz the power, 2014-09-01 14:18:01]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/joaodasilva@chromium.org/520703002/20001

[commit-bot: I haz the power, 2014-09-01 14:42:34]

Committed patchset #2 (id:20001) as a0158c3cc58ea93ba3a5de44df7452be5fb981a8

[commit-bot: I haz the power, 2014-09-10 03:16:56]

Patchset 2 (id:??) landed as
https://crrev.com/8d2d9254a094cb485874336117c8ca7960296c3d
Cr-Commit-Position: refs/heads/master@{#292874}