Persist sessionStorage on disk.

content/public/browser/session_storage_namespace.h

Index: content/public/browser/session_storage_namespace.h
diff --git a/content/public/browser/session_storage_namespace.h b/content/public/browser/session_storage_namespace.h
index 5b17d0ceaebd56e8c45794931e5525c6d17d47f8..e333c468007e3a5338655efebc64f1cc5448511d 100644
--- a/content/public/browser/session_storage_namespace.h
+++ b/content/public/browser/session_storage_namespace.h
@@ -6,6 +6,8 @@
 #define CONTENT_PUBLIC_BROWSER_SESSION_STORAGE_NAMESPACE_H_
 #pragma once
 
+#include <string>
+
 #include "base/memory/ref_counted.h"
 
 namespace content {
@@ -16,6 +18,9 @@ namespace content {
 //       it keeps valgrind happy.
 class SessionStorageNamespace
     : public base::RefCountedThreadSafe<SessionStorageNamespace> {
+ public:
+  virtual int64 id() const = 0;
+  virtual std::string persistent_id() const = 0;
  protected:
   friend class base::RefCountedThreadSafe<SessionStorageNamespace>;
   virtual ~SessionStorageNamespace() {}