[Sync] Read last poll time from correct pref.

Bug: 482154
Change-Id: I634489e826f6f92d45977d438a6e1f39e862a330
Reviewed-on: https://chromium-review.googlesource.com/677585Reviewed-by: Nicolas Zea <zea@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504462}

components/sync/base/sync_prefs.cc

@@ -173,7 +173,7 @@ void SyncPrefs::SetLastSyncedTime(base::Time time) {
 base::Time SyncPrefs::GetLastPollTime() const {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   return base::Time::FromInternalValue(
-      pref_service_->GetInt64(prefs::kSyncLastSyncedTime));
+      pref_service_->GetInt64(prefs::kSyncLastPollTime));
 }
 
 void SyncPrefs::SetLastPollTime(base::Time time) {