Revert "device/fido: add a cache for authenticator ECDH keys."

This reverts commit 5640f6c5.

Reason for revert: breaks PIN entry in some cases (crbug.com/1129713)

Original change's description:
> device/fido: add a cache for authenticator ECDH keys.
>
> While, at the moment, I don't believe we will end up asking for the
> authenticator's ECDH key twice, when we support the PRF extension we
> could. Thus add a cache and skip asking an authenticator for it twice.
>
> Change-Id: I73ecaa38447cbbe04f34c12d83f9c79bca553c90
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255208
> Commit-Queue: Adam Langley <agl@chromium.org>
> Auto-Submit: Adam Langley <agl@chromium.org>
> Reviewed-by: Martin Kreichgauer <martinkr@google.com>
> Cr-Commit-Position: refs/heads/master@{#781110}

Change-Id: Ief5dd139f1221b8bfbe58a6edf2390a022971538
Bug: 1129713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418852Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#808430}

device/fido/fido_device_authenticator.cc

@@ -208,12 +208,6 @@ void FidoDeviceAuthenticator::GetPinRetries(GetRetriesCallback callback) {
 
 void FidoDeviceAuthenticator::GetEphemeralKey(
     GetEphemeralKeyCallback callback) {
-  if (cached_ephemeral_key_.has_value()) {
-    std::move(callback).Run(CtapDeviceResponseCode::kSuccess,
-                            cached_ephemeral_key_);
-    return;
-  }
-
   DCHECK(Options());
   DCHECK(
       Options()->client_pin_availability !=
@@ -221,27 +215,10 @@ void FidoDeviceAuthenticator::GetEphemeralKey(
       Options()->supports_pin_uv_auth_token || SupportsHMACSecretExtension());
 
   RunOperation<pin::KeyAgreementRequest, pin::KeyAgreementResponse>(
-      pin::KeyAgreementRequest(),
-      base::BindOnce(&FidoDeviceAuthenticator::OnHaveEphemeralKey,
-                     weak_factory_.GetWeakPtr(), std::move(callback)),
+      pin::KeyAgreementRequest(), std::move(callback),
       base::BindOnce(&pin::KeyAgreementResponse::Parse));
 }
 
-void FidoDeviceAuthenticator::OnHaveEphemeralKey(
-    GetEphemeralKeyCallback callback,
-    CtapDeviceResponseCode status,
-    base::Optional<pin::KeyAgreementResponse> key) {
-  if (status != CtapDeviceResponseCode::kSuccess) {
-    std::move(callback).Run(status, base::nullopt);
-    return;
-  }
-  DCHECK(key.has_value());
-
-  cached_ephemeral_key_.emplace(std::move(key.value()));
-  std::move(callback).Run(CtapDeviceResponseCode::kSuccess,
-                          cached_ephemeral_key_);
-}
-
 void FidoDeviceAuthenticator::GetPINToken(
     std::string pin,
     const std::vector<pin::Permissions>& permissions,

device/fido/fido_device_authenticator.h

@@ -146,9 +146,6 @@ class COMPONENT_EXPORT(DEVICE_FIDO) FidoDeviceAuthenticator
                               base::Optional<pin::KeyAgreementResponse>)>;
   void InitializeAuthenticatorDone(base::OnceClosure callback);
   void GetEphemeralKey(GetEphemeralKeyCallback callback);
-  void OnHaveEphemeralKey(GetEphemeralKeyCallback callback,
-                          CtapDeviceResponseCode status,
-                          base::Optional<pin::KeyAgreementResponse> key);
   void DoGetAssertion(CtapGetAssertionRequest request,
                       CtapGetAssertionOptions options,
                       GetAssertionCallback callback);
@@ -251,7 +248,6 @@ class COMPONENT_EXPORT(DEVICE_FIDO) FidoDeviceAuthenticator
   base::Optional<AuthenticatorSupportedOptions> options_;
   std::unique_ptr<FidoTask> task_;
   std::unique_ptr<GenericDeviceOperation> operation_;
-  base::Optional<pin::KeyAgreementResponse> cached_ephemeral_key_;
   base::WeakPtrFactory<FidoDeviceAuthenticator> weak_factory_{this};
 
   DISALLOW_COPY_AND_ASSIGN(FidoDeviceAuthenticator);