Refactors offline page tab helper to stash the offline page item on redirect.

components/offline_pages/offline_page_item.h

Index: components/offline_pages/offline_page_item.h
diff --git a/components/offline_pages/offline_page_item.h b/components/offline_pages/offline_page_item.h
index 469c822212d4b9a5ecdf582c834920baebb3489b..f8970a5758d84ae1c719f07bc9566e0f24a69901 100644
--- a/components/offline_pages/offline_page_item.h
+++ b/components/offline_pages/offline_page_item.h
@@ -11,6 +11,7 @@
 
 #include "base/files/file_path.h"
 #include "base/strings/string16.h"
+#include "base/supports_user_data.h"
 #include "base/time/time.h"
 #include "url/gurl.h"
 
@@ -29,7 +30,7 @@ struct ClientId {
 };
 
 // Metadata of the offline page.
-struct OfflinePageItem {
+struct OfflinePageItem : public base::SupportsUserData::Data {
  public:
   // Note that this should match with Flags enum in offline_pages.proto.
   enum Flags {
@@ -50,7 +51,7 @@ struct OfflinePageItem {
                   int64_t file_size,
                   const base::Time& creation_time);
   OfflinePageItem(const OfflinePageItem& other);
-  ~OfflinePageItem();
+  ~OfflinePageItem() override;
 
   // Gets a URL of the file under |file_path|.
   GURL GetOfflineURL() const;