Add guard for crash in MemoryCache::pruneDeadResources

Add guard for crash in MemoryCache::pruneDeadResources

Although the root cause of crash is not clear now,
maybe we'd better add guard firstly to avoid crash.

BUG=594644
Committed: https://crrev.com/2ab370811b8850b96c579df7b545b901302b68df
Cr-Commit-Position: refs/heads/master@{#381761}

[leonhsl(Using Gerrit), 2016-03-17 06:18:14]

leon.han@intel.com changed reviewers:
+ mkwst@chromium.org

[leonhsl(Using Gerrit), 2016-03-17 06:18:15]

Hi Mike, Would you PTAL? 
I've already uploaded another CL trying to solve this crash from
//components/web_cache side, but I'm not sure it will fix it completely.. 
I have no knowledge of how the blink:WebCache is running specifically so we have
not gotten the root cause clearly now. So maybe we'd better guard the crash
firstly? Thanks~

[Mike West, 2016-03-17 16:20:11]

LGTM, but please add a TODO noting that we shouldn't be hitting this case, and
we'd like to turn it into an ASSERT.

[leonhsl(Using Gerrit), 2016-03-17 16:50:53]

On 2016/03/17 16:20:11, Mike West wrote:
> LGTM, but please add a TODO noting that we shouldn't be hitting this case, and
> we'd like to turn it into an ASSERT.

Thanks a lot for kindly approval~ Added TODO note and will land the CL now.
Thanks~

[leonhsl(Using Gerrit), 2016-03-17 16:51:11]

The CQ bit was checked by leon.han@intel.com

[leonhsl(Using Gerrit), 2016-03-17 16:51:11]

The patchset sent to the CQ was uploaded after l-g-t-m from mkwst@chromium.org
Link to the patchset: https://codereview.chromium.org/1813673002/#ps20001
(title: "Add TODO note")

[commit-bot: I haz the power, 2016-03-17 16:51:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1813673002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1813673002/20001

[commit-bot: I haz the power, 2016-03-17 18:46:18]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-03-17 18:48:17]

Description was changed from

==========
Add guard for crash in MemoryCache::pruneDeadResources

Although the root cause of crash is not clear now,
maybe we'd better add guard firstly to avoid crash.

BUG=594644
==========

to

==========
Add guard for crash in MemoryCache::pruneDeadResources

Although the root cause of crash is not clear now,
maybe we'd better add guard firstly to avoid crash.

BUG=594644

Committed: https://crrev.com/2ab370811b8850b96c579df7b545b901302b68df
Cr-Commit-Position: refs/heads/master@{#381761}
==========

[commit-bot: I haz the power, 2016-03-17 18:48:18]

Patchset 2 (id:??) landed as
https://crrev.com/2ab370811b8850b96c579df7b545b901302b68df
Cr-Commit-Position: refs/heads/master@{#381761}