QUIC - Browser shutdown crash fix. Accessing disk_cache's backend after

QUIC - Browser shutdown crash fix. Accessing disk_cache's backend after
it is deleted.

Did a one line fix to reset callback and commented out code that collected
UMA histogram data. Will implement correct fix asap.

TBR=rch@chromium.org
BUG=456524
Committed: https://crrev.com/4e6ea99ce0a36f7111f650e302672ada269f78b0
Cr-Commit-Position: refs/heads/master@{#315243}

[ramant (doing other things), 2015-02-08 20:29:33]

The CQ bit was checked by rtenneti@chromium.org

[commit-bot: I haz the power, 2015-02-08 20:30:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/892083006/1

[ramant (doing other things), 2015-02-08 20:38:24]

The CQ bit was checked by rtenneti@chromium.org

[commit-bot: I haz the power, 2015-02-08 20:38:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/892083006/10003

[ramant (doing other things), 2015-02-08 20:39:42]

Patchset #1 (id:1) has been deleted

[commit-bot: I haz the power, 2015-02-08 21:36:29]

Committed patchset #2 (id:10003)

[commit-bot: I haz the power, 2015-02-08 21:36:57]

Patchset 1 (id:??) landed as
https://crrev.com/4e6ea99ce0a36f7111f650e302672ada269f78b0
Cr-Commit-Position: refs/heads/master@{#315243}