[webauthn] Obfuscate Safetynet nonce

leshi has just created a new issue for https://github.com/w3c/webauthn:

== Obfuscate Safetynet nonce ==
The current signing procedure for the Safetynet attestation states:

> Let authenticatorData denote the authenticator data for the attestation, and let clientDataHash denote the hash of the serialized client data.
> 
> Concatenate authenticatorData and clientDataHash to form attToBeSigned.
> 
> Request a SafetyNet attestation, providing attToBeSigned as the nonce value.

This has the product of sending unnecessary data to the Safetynet servers.  For example, ```rpIdHash``` would be easy to brute-force.

Instead, we should be sending the hash of ```attToBeSigned.```

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/868 using your GitHub account