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

chrome/browser/thumbnails/thumbnail_list_source.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/thumbnails/thumbnail_list_source.h"
 
 #include <string>
 
 #include "base/base64.h"
 #include "base/bind.h"
@@ skipping 76 matching lines @@
 
 std::string ThumbnailListSource::GetSource() const {
   return chrome::kChromeUIThumbnailListHost;
 }
 
 void ThumbnailListSource::StartDataRequest(
     const std::string& path,
     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(),
                  callback), true);
 }
 
 std::string ThumbnailListSource::GetMimeType(const std::string& path) const {
   return "text/html";
 }
 
@@ skipping 42 matching lines @@
   }
   if (num_mv_with_thumb < num_mv) {
     out.push_back("<h2>TopSites URLs without Thumbnails</h2>\n");
     RenderMostVisitedURLList(mvurl_list, base64_encoded_pngs, false, &out);
   }
   out.push_back(kHtmlFooter);
 
   std::string out_html = JoinString(out, "");
   callback.Run(base::RefCountedString::TakeString(&out_html));
 }