Fix NTP thumbnail generation

chrome/browser/thumbnails/thumbnailing_context.h

Index: chrome/browser/thumbnails/thumbnailing_context.h
diff --git a/chrome/browser/thumbnails/thumbnailing_context.h b/chrome/browser/thumbnails/thumbnailing_context.h
index 1cfe67178aeb2a2270510a0da26cb12058efddb8..bd59047c0895a32f1004b5cb53fbc5db149a2a1b 100644
--- a/chrome/browser/thumbnails/thumbnailing_context.h
+++ b/chrome/browser/thumbnails/thumbnailing_context.h
@@ -10,6 +10,7 @@
 #include "chrome/browser/thumbnails/thumbnail_service.h"
 #include "components/history/core/common/thumbnail_score.h"
 #include "content/public/browser/web_contents.h"
+#include "content/public/browser/web_contents_observer.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace thumbnails {
@@ -34,7 +35,9 @@ enum ClipResult {
 };
 
 // Holds the information needed for processing a thumbnail.
-struct ThumbnailingContext : base::RefCountedThreadSafe<ThumbnailingContext> {
+struct ThumbnailingContext

[Lei Zhang, 2015/10/26 23:31:35]

This is looking a bit more complicated now. When in doubt, make it a class.

+    : base::RefCountedThreadSafe<ThumbnailingContext>,
+      content::WebContentsObserver {
   ThumbnailingContext(content::WebContents* web_contents,
                       ThumbnailService* receiving_service,
                       bool load_interrupted);