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

chrome/browser/thumbnails/thumbnail_list_source.cc

Index: chrome/browser/thumbnails/thumbnail_list_source.cc
diff --git a/chrome/browser/thumbnails/thumbnail_list_source.cc b/chrome/browser/thumbnails/thumbnail_list_source.cc
index ad47041b0fa4fbf9ac46477105c93a318028cdf9..bd447c20a6bd1f213e1f932223a4b7533dfbd57b 100644
--- a/chrome/browser/thumbnails/thumbnail_list_source.cc
+++ b/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(),