[Suggestions Component] Add missing NULL check to ImageManager::OnImageFetched().

[Suggestions Component] Add missing NULL check to ImageManager::OnImageFetched().

This fixes crashing bug. The check should be in ImageManager::OnImageFetched()
instead of ImageFetcherImpl::OnFetchComplete(). Indeed, the unit test in
image_fetcher_impl_browsertest.cc also allows it.

Also added extra non-NULL DCHECK in ImageManager::GetImageURL(), and
verified that current callers do not passing NULL to it.

BUG=428117
TBR=mathp

Committed: https://crrev.com/53313374f1ee883f825be0ac5f879d171be57a29
Cr-Commit-Position: refs/heads/master@{#302258}

[huangs, 2014-10-30 23:00:23]

huangs@chromium.org changed reviewers:
+ beaudoin@chromium.org, mathp@chromium.org

[huangs, 2014-10-30 23:00:23]

PTAL.

[beaudoin, 2014-10-30 23:03:25]

lgtm

[huangs, 2014-10-30 23:36:47]

The CQ bit was checked by huangs@chromium.org

[commit-bot: I haz the power, 2014-10-30 23:37:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/689993004/1

[huangs, 2014-10-31 13:09:20]

The CQ bit was unchecked by huangs@chromium.org

[huangs, 2014-10-31 13:10:18]

The CQ bit was checked by huangs@chromium.org

[commit-bot: I haz the power, 2014-10-31 13:11:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/689993004/1

[commit-bot: I haz the power, 2014-10-31 13:59:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-31 14:00:13]

Patchset 1 (id:??) landed as
https://crrev.com/53313374f1ee883f825be0ac5f879d171be57a29
Cr-Commit-Position: refs/heads/master@{#302258}

[Mathieu, 2014-11-04 18:28:29]

On 2014/10/31 14:00:13, I haz the power (commit-bot) wrote:
> Patchset 1 (id:??) landed as
> https://crrev.com/53313374f1ee883f825be0ac5f879d171be57a29
> Cr-Commit-Position: refs/heads/master@{#302258}

lgtm, thanks!