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

content/public/browser/media_device_id.cc

Index: content/public/browser/media_device_id.cc
diff --git a/content/public/browser/media_device_id.cc b/content/public/browser/media_device_id.cc
index c2e2c3c09652960d97fcaf886493cd67bb35d84b..a67f49b1c049abe71e111b1ab7a985eef284456f 100644
--- a/content/public/browser/media_device_id.cc
+++ b/content/public/browser/media_device_id.cc
@@ -6,30 +6,36 @@
 #include "base/logging.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
+#include "content/public/browser/content_browser_client.h"
+#include "content/public/common/content_client.h"
 #include "crypto/hmac.h"
 
 namespace content {
 
-std::string GetHMACForMediaDeviceID(const GURL& security_origin,
+std::string GetHMACForMediaDeviceID(ResourceContext* rc,
+                                    const GURL& security_origin,
                                     const std::string& raw_unique_id) {
   DCHECK(security_origin.is_valid());
   DCHECK(!raw_unique_id.empty());
   crypto::HMAC hmac(crypto::HMAC::SHA256);
   const size_t digest_length = hmac.DigestLength();
   std::vector<uint8> digest(digest_length);
+  const std::string& salt = GetContentClient()->browser()->GetDeviceIdSalt(rc);
+  DCHECK(!salt.empty());
   bool result = hmac.Init(security_origin.spec()) &&
-      hmac.Sign(raw_unique_id, &digest[0], digest.size());
+      hmac.Sign(raw_unique_id + salt, &digest[0], digest.size());
   DCHECK(result);
   return StringToLowerASCII(base::HexEncode(&digest[0], digest.size()));
 }
 
-bool DoesMediaDeviceIDMatchHMAC(const GURL& security_origin,
+bool DoesMediaDeviceIDMatchHMAC(ResourceContext* rc,
+                                const GURL& security_origin,
                                 const std::string& device_guid,
                                 const std::string& raw_unique_id) {
   DCHECK(security_origin.is_valid());
   DCHECK(!raw_unique_id.empty());
   std::string guid_from_raw_device_id =
-      GetHMACForMediaDeviceID(security_origin, raw_unique_id);
+      GetHMACForMediaDeviceID(rc, security_origin, raw_unique_id);
   return guid_from_raw_device_id == device_guid;
 }