Implement a salt for MediaSource IDs that can be cleared by a user.

chrome/browser/media/media_device_id_salt.h

Index: chrome/browser/media/media_device_id_salt.h
diff --git a/chrome/browser/media/media_device_id_salt.h b/chrome/browser/media/media_device_id_salt.h
new file mode 100644
index 0000000000000000000000000000000000000000..7e117271cb31261fb64f2cb807a288c6deb80a39
--- /dev/null
+++ b/chrome/browser/media/media_device_id_salt.h
@@ -0,0 +1,36 @@
+// Copyright 2013 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 CHROME_BROWSER_MEDIA_MEDIA_DEVICE_ID_SALT_H_
+#define CHROME_BROWSER_MEDIA_MEDIA_DEVICE_ID_SALT_H_
+
+#include <string>
+
+#include "base/prefs/pref_member.h"
+#include "components/user_prefs/pref_registry_syncable.h"
+
+class PrefService;
+
+// MediaDeviceIDSalt is responsible for creating and retrieving a salt string
+// that is used for creating MediaSource IDs that can be cached by a web
+// service. If the cache is cleared, the  MediaSourceIds are invalidated.
+class MediaDeviceIDSalt {
+ public:
+  MediaDeviceIDSalt(PrefService* pref_service,
+                    bool incognito);
+  ~MediaDeviceIDSalt();
+
+  std::string GetSalt() const;
+
+  static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
+  static void Reset(PrefService* pref_service);
+
+ private:
+  // |incognito_salt_| is initialized in ctor on UI thread but only read
+  //  on the IO thread.
+  std::string incognito_salt_;
+  mutable StringPrefMember media_device_id_salt_;
+};
+
+#endif  // CHROME_BROWSER_MEDIA_MEDIA_DEVICE_ID_SALT_H_