Make default media device ID salts random by default

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 <stdint.h>
 
 #include <memory>
@@ skipping 36 matching lines @@
 
   // Create a platform KeygenHandler and pass it to |callback|. The |callback|
   // may be run synchronously.
   virtual void CreateKeygenHandler(
       uint32_t key_size_in_bits,
       const std::string& challenge_string,
       const GURL& url,
       const base::Callback<void(std::unique_ptr<net::KeygenHandler>)>&
           callback);
 
-  // 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(joi): We don't think it should be unnecessary to use this
-  // after ResourceContext goes away. There is likely an underying bug
-  // in the lifetime of ProfileIOData vs. ResourceProcessHost, where
-  // sometimes ProfileIOData has gone away before RPH has finished
-  // being torn down (on the IO thread). The current interface that
-  // allows using the salt object after ResourceContext has gone away
-  // was put in place to fix http://crbug.com/341211 but I intend to
-  // try to figure out how the lifetime should be fixed properly. The
-  // original interface was just a method that returns a string.
-  //
-  // TODO(perkj): Make this method pure virtual when crbug/315022 is
-  // fixed.
-  typedef base::Callback<std::string()> SaltCallback;
-  virtual SaltCallback GetMediaDeviceIDSalt();
+  // 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

[jam, 2016/06/14 16:20:33]

nit: don't mention user profile, since that's a chrome concept

[Guido Urdaneta, 2016/06/14 17:30:57]

Done.

+  // if cookies are cleared. Returns a random string by default.
+  virtual const std::string GetMediaDeviceIDSalt();
+
+  // Utility function useful for implementations

[jam, 2016/06/14 16:20:33]

nit: make it explicit that this only needs to be called if 
1) the embedder needs to use a new salt 
2) the embedder saves its salt across restart

[Guido Urdaneta, 2016/06/14 17:30:57]

Done.

+  static std::string CreateRandomMediaDeviceIDSalt();
+
+ private:
+  const std::string media_device_id_salt_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_RESOURCE_CONTEXT_H_