[Offline Pages] Adding missing image/CSS detection in FrameSerializer.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index bf90b0ae15404e326ea89b1e84085293b60788ed..ab0ea06946cb4ffa4b4fb97145843f1071d7f803 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -697,6 +697,10 @@ class MHTMLPartsGenerationDelegate
     return params_.mhtml_popup_overlay_removal;
   }
 
+  bool UsePageProblemDetectors() override {
+    return params_.mhtml_problem_detection;
+  }
+
  private:
   const FrameMsg_SerializeAsMHTML_Params& params_;
   std::set<std::string>* serialized_resources_uri_digests_;