Fix NTP thumbnail generation

chrome/browser/thumbnails/thumbnailing_context.cc

 // Copyright (c) 2013 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/thumbnailing_context.h"
 
 #include "content/public/browser/render_view_host.h"
 #include "content/public/browser/render_widget_host_view.h"
 
 namespace thumbnails {
 
 ThumbnailingContext::ThumbnailingContext(content::WebContents* web_contents,
                                          ThumbnailService* receiving_service,
                                          bool load_interrupted)
-    : service(receiving_service),
+    : content::WebContentsObserver(web_contents),
+      service(receiving_service),
       url(web_contents->GetURL()),
       clip_result(CLIP_RESULT_UNPROCESSED) {
   score.at_top =
       (web_contents->GetRenderWidgetHostView()->GetLastScrollOffset().y() == 0);
   score.load_completed = !web_contents->IsLoading() && !load_interrupted;
 }
 
 ThumbnailingContext::ThumbnailingContext()
   : clip_result(CLIP_RESULT_UNPROCESSED) {
 }
 
 ThumbnailingContext::~ThumbnailingContext() {
 }
 
 }  // namespace thumbnails