Fix crash in ZeroStateFileProvider.

When a FilesTaskNotifier is not available (which happens sometimes in
guest mode), the |files_ranker_| object in ZeroStateFileProvider never
gets initialized, crashing Chrome the next time it is used.

Bug: 1012911
Change-Id: Ic1d2231d4d3e9810fd16a7425a98faf94c65115e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850672
Auto-Submit: Tony Yeoman <tby@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Commit-Queue: Charles . <charleszhao@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704491}

chrome/browser/ui/app_list/search/zero_state_file_provider.cc

@@ -79,7 +79,7 @@ ZeroStateFileProvider::~ZeroStateFileProvider() = default;
 void ZeroStateFileProvider::Start(const base::string16& query) {
   query_start_time_ = base::TimeTicks::Now();
   ClearResultsSilently();
-  if (!query.empty())
+  if (!files_ranker_ || !query.empty())
     return;
 
   base::PostTaskAndReplyWithResult(