Fix NTP thumbnail generation

chrome/browser/thumbnails/thumbnail_tab_helper.cc

Index: chrome/browser/thumbnails/thumbnail_tab_helper.cc
diff --git a/chrome/browser/thumbnails/thumbnail_tab_helper.cc b/chrome/browser/thumbnails/thumbnail_tab_helper.cc
index f8322e76f9a2d9573939dea942a32fe342b8bffb..c26fd5febe4dbb083576383c474ae1a90ec35269 100644
--- a/chrome/browser/thumbnails/thumbnail_tab_helper.cc
+++ b/chrome/browser/thumbnails/thumbnail_tab_helper.cc
@@ -66,6 +66,13 @@ void ProcessCapturedBitmap(scoped_refptr<ThumbnailingContext> context,
                            scoped_refptr<ThumbnailingAlgorithm> algorithm,
                            const SkBitmap& bitmap,
                            content::ReadbackResponse response) {
+  // Was the web contents destroyed before the thumbnail could be generated?
+  if (!context->web_contents())
+    return;
+
+  // Balance the IncrementCapturerCount() from UpdateThumbnailIfNecessary().
+  context->web_contents()->DecrementCapturerCount();
+
   if (response != content::READBACK_SUCCESS)
     return;
 
@@ -76,11 +83,11 @@ void ProcessCapturedBitmap(scoped_refptr<ThumbnailingContext> context,
   algorithm->ProcessBitmap(context, base::Bind(&UpdateThumbnail), bitmap);
 }
 
-void AsyncProcessThumbnail(content::WebContents* web_contents,
-                           scoped_refptr<ThumbnailingContext> context,
+void AsyncProcessThumbnail(scoped_refptr<ThumbnailingContext> context,
                            scoped_refptr<ThumbnailingAlgorithm> algorithm) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
-  RenderWidgetHost* render_widget_host = web_contents->GetRenderViewHost();
+  RenderWidgetHost* render_widget_host =
+      context->web_contents()->GetRenderViewHost();
   content::RenderWidgetHostView* view = render_widget_host->GetView();
   if (!view)
     return;
@@ -196,12 +203,14 @@ void ThumbnailTabHelper::UpdateThumbnailIfNecessary(
     return;
   }
 
+  web_contents->IncrementCapturerCount(gfx::Size());
+
   scoped_refptr<thumbnails::ThumbnailingAlgorithm> algorithm(
       thumbnail_service->GetThumbnailingAlgorithm());
 
   scoped_refptr<ThumbnailingContext> context(new ThumbnailingContext(
       web_contents, thumbnail_service.get(), load_interrupted_));
-  AsyncProcessThumbnail(web_contents, context, algorithm);
+  AsyncProcessThumbnail(context, algorithm);
 }
 
 void ThumbnailTabHelper::RenderViewHostCreated(