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

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 7c553eb95b6e1fca05fe49b60d581ab8ab48e536..8f449e0b0ac7b41281375123adfc9f6b1ce13f15 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -1117,6 +1117,9 @@ const char kDefaultAudioCaptureDevice[] = "media.default_audio_capture_device";
 // The default video capture device used by the Media content setting.
 const char kDefaultVideoCaptureDevice[] = "media.default_video_capture_Device";
 
+// The salt used for creating random MediaSource IDs.
+const char kMediaDeviceIdSalt[] = "media.device_id_salt";
+
 // Preference to disable 3D APIs (WebGL, Pepper 3D).
 const char kDisable3DAPIs[] = "disable_3d_apis";