Fix large blob key extension ID

https://github.com/fido-alliance/fido-2-specs/pull/1036 fixed the member
ID for large blob key on GetAssertion. The spec used to quote both 0x0b
and 0x07, and I originally implemented the one that didn't get chosen.

This feature is guarded behind the WebAuthenticationLargeBlobExtension
feature flag.

https://w3c.github.io/webauthn/#sctn-large-blob-extension

Bug: 1114875
Change-Id: I7ae04dbb35b5df586271737858071b6ee5601cd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468639
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#816782}

device/fido/device_response_converter.cc

@@ -190,7 +190,7 @@ base::Optional<AuthenticatorGetAssertionResponse> ReadCTAPGetAssertionResponse(
     }
   }
 
-  it = response_map.find(CBOR(0x0B));
+  it = response_map.find(CBOR(7));
   if (it != response_map.end()) {
     if (!it->second.is_bytestring() ||
         it->second.GetBytestring().size() != kLargeBlobKeyLength) {

device/fido/virtual_ctap2_device.cc

@@ -438,7 +438,7 @@ std::vector<uint8_t> EncodeGetAssertionResponse(
                          cbor::Value(*response.android_client_data_ext()));
   }
   if (response.large_blob_key()) {
-    response_map.emplace(0x0b, cbor::Value(*response.large_blob_key()));
+    response_map.emplace(0x07, cbor::Value(*response.large_blob_key()));
   }
 
   return WriteCBOR(cbor::Value(std::move(response_map)), allow_invalid_utf8);