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

content/test/webrtc_audio_device_test.cc

Index: content/test/webrtc_audio_device_test.cc
diff --git a/content/test/webrtc_audio_device_test.cc b/content/test/webrtc_audio_device_test.cc
index 641406f0bcc695b2b2a9e0d262944785c019e6f5..14595413698348e20a59d4cd1be86ca8bc6719ba 100644
--- a/content/test/webrtc_audio_device_test.cc
+++ b/content/test/webrtc_audio_device_test.cc
@@ -174,6 +174,10 @@ class MockRTCResourceContext : public ResourceContext {
     return false;
   }
 
+  virtual std::string GetMediaDeviceIDSalt() OVERRIDE {
+    return "";
+  }
+
  private:
   net::URLRequestContext* test_request_context_;