[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

Review URL: https://codereview.chromium.org/501973004

Cr-Commit-Position: refs/heads/master@{#292034}
(cherry picked from commit 94a4210d30dfa1201a818dc76d6a7afda1baf47d)

Review URL: https://codereview.chromium.org/533663002

Cr-Commit-Position: 0d344d3782ceeadf0e8e8be3f4ce70193b218c54@{#173}
Cr-Branched-From: 2cb3f0147827a1600e8cbbcc52ca24c0febcb7a4@{#290040}

ondra-novak/chromium.src,littlstar/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,littlstar/chromium.src,littlstar/chromium.src,littlstar/chromium.src,littlstar/chromium.src,littlstar/chromium.src,littlstar/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,littlstar/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src