chrome/browser/webauthn: fix warning

Change-Id: Ibf77d613bb5de9fdbcbbfef94746106cbf3004f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593877
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#656422}

chrome/browser/webauthn/authenticator_request_dialog_model.h

@@ -125,7 +125,7 @@ class AuthenticatorRequestDialogModel {
     virtual void OnCancelRequest() {}
   };
 
-  AuthenticatorRequestDialogModel(const std::string& relying_party_id);
+  explicit AuthenticatorRequestDialogModel(const std::string& relying_party_id);
   ~AuthenticatorRequestDialogModel();
 
   void SetCurrentStep(Step step);