fido: log FidoDevice::Cancel() invocations to chrome://device-log

This helps with reasoning from the logs whether an outstanding CTAP2
request was cancelled, e.g. by sending CTAPHID_CANCEL.

Change-Id: I3c93cbd399a074f03821cebcfa2825b6d96b3f2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949037
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Adam Langley <agl@chromium.org>
Auto-Submit: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721150}

device/fido/ctap2_device_operation.h

@@ -109,6 +109,7 @@ class Ctap2DeviceOperation : public DeviceOperation<Request, Response> {
   // |kCtap2ErrKeepAliveCancel|.
   void Cancel() override {
     if (this->token_) {
+      FIDO_LOG(DEBUG) << "<- (cancel)";
       this->device()->Cancel(*this->token_);
       this->token_.reset();
     }