Introduce CertErrors::ToDebugString().

net/cert/internal/cert_errors.h

Index: net/cert/internal/cert_errors.h
diff --git a/net/cert/internal/cert_errors.h b/net/cert/internal/cert_errors.h
index 4dd86a10cd5302e52128821bf3870cf1098adb80..f12133b06350b232630172eed538bc0f5929c26c 100644
--- a/net/cert/internal/cert_errors.h
+++ b/net/cert/internal/cert_errors.h
@@ -115,6 +115,9 @@ class NET_EXPORT CertErrors {
 
   const std::vector<CertError>& errors() const { return errors_; }
 
+  // Dumps a textual representation of the errors for debugging purposes.
+  std::string ToDebugString() const;
+
  private:
   std::vector<CertError> errors_;