omnibox: fix possible crash

This is a regression from:
https://chromium-review.googlesource.com/c/chromium/src/+/2483342

The case under which the crash happens is when the internal cache
(URLIndexPrivateData) is being rebuilt on the history thread and
GetMostRecentVisitsForURL() returns false. I believe that basically
only happens if there is a problem with the db.

I would have liked a test for this, but there is no way to
force the db to return false.

BUG=1146970
TEST=none

Change-Id: Ied6a7e1c75ef8fcc47043d231f4877a597757bf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527006Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825464}

components/omnibox/browser/url_index_private_data.cc

@@ -937,9 +937,8 @@ bool URLIndexPrivateData::IndexRow(
   // as appropriate.
   if (got_visits) {
     UpdateRecentVisits(row_id, recent_visits);
-  } else {
+  } else if (history_service) {
     DCHECK(tracker);
-    DCHECK(history_service);
     ScheduleUpdateRecentVisits(history_service, row_id, tracker);
   }