[Sync] Fix a crash when sign into sync.

[Sync] Fix a crash when sign into sync.

The crash was caused by a dangling pointer to a deleted observer
registered by MostVisitedSites.

BUG=406996
Committed: https://crrev.com/94a4210d30dfa1201a818dc76d6a7afda1baf47d
Cr-Commit-Position: refs/heads/master@{#292034}

[Feng Qian, 2014-08-26 20:56:23]

feng@chromium.org changed reviewers:
+ mathp@chromium.org, yfriedman@chromium.org

[Mathieu, 2014-08-26 20:58:26]

lgtm, I'm so sorry :\

[Feng Qian, 2014-08-26 21:00:03]

On 2014/08/26 20:58:26, Mathieu Perreault wrote:
> lgtm, I'm so sorry :\

no problem.

Yaron, I think it still needs owner's approval. Can you take a look the change?

[Feng Qian, 2014-08-26 21:12:26]

feng@chromium.org changed reviewers:
+ newt@chromium.org

[newt (away), 2014-08-26 21:32:36]

lgtm

[Feng Qian, 2014-08-26 21:33:00]

The CQ bit was checked by feng@chromium.org

[commit-bot: I haz the power, 2014-08-26 21:35:25]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/feng@chromium.org/501973004/1

[Yaron, 2014-08-26 21:53:41]

lgtm

[commit-bot: I haz the power, 2014-08-26 23:43:14]

Committed patchset #1 (1) as 84c031ded14aaf5944fc2561ab5102fe5cd3534d

[commit-bot: I haz the power, 2014-09-10 02:47:28]

Patchset 1 (id:??) landed as
https://crrev.com/94a4210d30dfa1201a818dc76d6a7afda1baf47d
Cr-Commit-Position: refs/heads/master@{#292034}