[Guest mode] Make sure chrome://thumbnails doesn't crash.

Checking for existence of TopSites.

BUG=407527

Review URL: https://codereview.chromium.org/507653002

Cr-Commit-Position: refs/heads/master@{#292010}

chrome/browser/thumbnails/thumbnail_list_source.cc

@@ -94,6 +94,11 @@ void ThumbnailListSource::StartDataRequest(
     int render_process_id,
     int render_frame_id,
     const content::URLDataSource::GotDataCallback& callback) {
+  if (!profile_->GetTopSites()) {
+    callback.Run(NULL);
+    return;
+  }
+
   profile_->GetTopSites()->GetMostVisitedURLs(
       base::Bind(&ThumbnailListSource::OnMostVisitedURLsAvailable,
                  weak_ptr_factory_.GetWeakPtr(),