Make Fiddler CHACHA20_POLY1305 aware.

CustomRules.js

//Add these two imports at the top of 'CustomRules.js'
import System;
import System.Reflection;

//Create or add this to the 'OnBoot' function:

static function OnBoot() : void {
  var ciphersField = FiddlerApplication.Assembly.GetType("Fiddler.HTTPSClientHello").GetField("dictTLSCipherSuites", BindingFlags.NonPublic | BindingFlags.Static);
  var ciphers = ciphersField.GetValue(null);
  ciphers.set_Item(0xCCA8, "TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256");
  ciphers.set_Item(0xCCA9, "TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256");
  ciphers.set_Item(0xCCAA, "TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256");
  ciphers.set_Item(0xCCAB, "TLS_PSK_WITH_CHACHA20_POLY1305_SHA256");
  ciphers.set_Item(0xCCAC, "TLS_ECDHE_PSK_WITH_CHACHA20_POLY1305_SHA256");
  ciphers.set_Item(0xCCAD, "TLS_DHE_PSK_WITH_CHACHA20_POLY1305_SHA256");
  ciphers.set_Item(0xCCAE, "TLS_RSA_PSK_WITH_CHACHA20_POLY1305_SHA256");
  ciphers.set_Item(0xCC14, "OLD_" + ciphers.get_Item(0xCC14));
  ciphers.set_Item(0xCC13, "OLD_" + ciphers.get_Item(0xCC13));
}

//Restart Fiddler