Prepare ExternalPolicyDataUpdater and ResourceCache for blocking pool

Prepare ExternalPolicyDataUpdater and ResourceCache for blocking pool

This CL allows the ExternalPolicyDataUpdater and ResourceCache classes to
be used in the blocking pool. It is a prerequisite for CL 22562003 which
will add code to downloading and caching of external policy data that runs
in the blocking pool and uses these classes.

BUG=256635
TEST=Updated unit tests

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=223340

[bartfab (slow), 2013-09-12 16:55:41]

Hi Joao,

Could you please review?

[bartfab (slow), 2013-09-12 16:56:33]

Hi Mattias,

Joao is very busy right now. Would you have the cycles to review?

[bartfab (slow), 2013-09-16 11:29:31]

Hi Daniel,

Since you offered to review - feel free to take a look :).

[Mattias Nissler (ping if slow), 2013-09-16 11:42:20]

LGTM

[commit-bot: I haz the power, 2013-09-16 12:16:31]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/bartfab@chromium.org/23868021/10001

[commit-bot: I haz the power, 2013-09-16 14:38:52]

Change committed as 223340