[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}

chrome/browser/android/most_visited_sites.cc

@@ -209,6 +209,10 @@ MostVisitedSites::MostVisitedSites(Profile* profile)
 }
 
 MostVisitedSites::~MostVisitedSites() {
+  ProfileSyncService* profile_sync_service =
+      ProfileSyncServiceFactory::GetForProfile(profile_);
+  if (profile_sync_service && profile_sync_service->HasObserver(this))
+    profile_sync_service->RemoveObserver(this);
 }
 
 void MostVisitedSites::Destroy(JNIEnv* env, jobject obj) {