Allow retrieval of media device ID salt even after ResourceContext has gone away.

content/public/browser/resource_context.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_RESOURCE_CONTEXT_H_
 #define CONTENT_PUBLIC_BROWSER_RESOURCE_CONTEXT_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 50 matching lines @@
   // Returns true if microphone access is allowed for |origin|. Used to
   // determine what level of authorization is given to |origin| to access
   // resource metadata.
   virtual bool AllowMicAccess(const GURL& origin) = 0;
 
   // Returns true if web camera access is allowed for |origin|. Used to
   // determine what level of authorization is given to |origin| to access
   // resource metadata.
   virtual bool AllowCameraAccess(const GURL& origin) = 0;
 
-  // Returns a random salt string that is used for creating media device IDs.
-  // The salt should be stored in the current user profile and should be reset
-  // if cookies are cleared. Returns an empty string per default.
-  // TODO(perkj): Make this method pure virtual when crbug/315022 is fixed.
-  virtual std::string GetMediaDeviceIDSalt();
+  // Returns a callback that can be invoked to get a random salt
+  // string that is used for creating media device IDs.  The salt
+  // should be stored in the current user profile and should be reset
+  // if cookies are cleared. The default is an empty string.
+  //
+  // It is safe to hold on to the callback returned and use it without
+  // regard to the lifetime of ResourceContext, although in general
+  // you should not use it long after the profile has been destroyed.
+  //
+  // TODO(perkj): Make this method pure virtual when crbug/315022 is
+  // fixed.
+  typedef base::Callback<std::string()> SaltCallback;
+  virtual SaltCallback GetMediaDeviceIDSalt();
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_RESOURCE_CONTEXT_H_