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..7cdfa3f9cf04e6e1dbb09c821b547c93c4b93b2f 100644
--- a/chrome/browser/thumbnails/thumbnail_tab_helper.cc
+++ b/chrome/browser/thumbnails/thumbnail_tab_helper.cc
@@ -66,6 +66,9 @@ void ProcessCapturedBitmap(scoped_refptr<ThumbnailingContext> context,
                            scoped_refptr<ThumbnailingAlgorithm> algorithm,
                            const SkBitmap& bitmap,
                            content::ReadbackResponse response) {
+  // Balance the IncrementCapturerCount() from UpdateThumbnailIfNecessary().
+  context->web_contents->DecrementCapturerCount();
+
   if (response != content::READBACK_SUCCESS)
     return;
 
@@ -196,6 +199,8 @@ void ThumbnailTabHelper::UpdateThumbnailIfNecessary(
     return;
   }
 
+  web_contents->IncrementCapturerCount(gfx::Size());
+
   scoped_refptr<thumbnails::ThumbnailingAlgorithm> algorithm(
       thumbnail_service->GetThumbnailingAlgorithm());