Reland of "Remove void** from disk_cache interface" w/ blockfile iter fix.

Reland of "Remove void** from disk_cache interface" w/ blockfile iter fix.

Enumeration and iteration were passing around void**. With this CL, we
instead use an Iterator object.

TBR=jkarlin@chromium.org,rvargas@chromium.org,clamy@chromium.org,piman@chromium.org
BUG=413644, 416059
TEST=None

Committed: https://crrev.com/b1aaa058d259c916a01b675ca297cc01c7a07b17
Cr-Commit-Position: refs/heads/master@{#296415}

[gavinp, 2014-09-22 16:42:30]

gavinp@chromium.org changed reviewers:
+ clamy@chromium.org, jkarlin@chromium.org, rvargas@chromium.org

[gavinp, 2014-09-22 16:44:37]

This CL incorporates the (now twice reverted) code from
https://codereview.chromium.org/583283002/ , along with the (already written,
DOH) fix to this issue, in https://codereview.chromium.org/583293002/ .

I'll watch the bots and go CQ once it's well baked.

[gavinp, 2014-09-23 19:47:23]

gavinp@chromium.org changed reviewers:
+ piman@chromium.org

[gavinp, 2014-09-24 01:45:43]

The CQ bit was checked by gavinp@chromium.org

[commit-bot: I haz the power, 2014-09-24 01:46:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/588243002/60001

[commit-bot: I haz the power, 2014-09-24 01:56:21]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-24 01:56:22]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[gavinp, 2014-09-24 14:48:53]

The CQ bit was checked by gavinp@chromium.org

[commit-bot: I haz the power, 2014-09-24 14:49:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/588243002/80001

[commit-bot: I haz the power, 2014-09-24 14:54:46]

Committed patchset #5 (id:80001) as 7192ea96dab00924762834cc11bb48df5f750e5b

[commit-bot: I haz the power, 2014-09-24 14:55:23]

Patchset 5 (id:??) landed as
https://crrev.com/b1aaa058d259c916a01b675ca297cc01c7a07b17
Cr-Commit-Position: refs/heads/master@{#296415}