Mandoline: fix the crash when a remote subframe tries to access session storage.

components/html_viewer/html_widget.h

Index: components/html_viewer/html_widget.h
diff --git a/components/html_viewer/html_widget.h b/components/html_viewer/html_widget.h
index 37074ce1c7ab25a65d848ade5dbf12bd6da260e1..21f7daadccf6e0ad2a6a5617872d93449cb6f8fc 100644
--- a/components/html_viewer/html_widget.h
+++ b/components/html_viewer/html_widget.h
@@ -38,12 +38,16 @@ class HTMLWidget {
 };
 
 // Used for the root frame when the root frame is remote.
-class HTMLWidgetRootRemote : public HTMLWidget {
+class HTMLWidgetRootRemote : public HTMLWidget, public blink::WebViewClient {
  public:
   HTMLWidgetRootRemote();
   ~HTMLWidgetRootRemote() override;
 
  private:
+  // WebViewClient methods:
+  virtual blink::WebStorageNamespace* createSessionStorageNamespace();
+  virtual bool allowsBrokenNullLayerTreeView() const;
+
   // HTMLWidget:
   blink::WebWidget* GetWidget() override;
   void OnViewBoundsChanged(mus::View* view) override;