Provide default implementation of QuicheLowerCaseString.

PiperOrigin-RevId: 448947268

quiche/common/platform/default/quiche_platform_impl/quiche_lower_case_string_impl.h

diff --git a/quiche/common/platform/default/quiche_platform_impl/quiche_lower_case_string_impl.h b/quiche/common/platform/default/quiche_platform_impl/quiche_lower_case_string_impl.h
new file mode 100644
index 0000000..856928a
--- /dev/null
+++ b/quiche/common/platform/default/quiche_platform_impl/quiche_lower_case_string_impl.h
@@ -0,0 +1,25 @@
+#ifndef QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUICHE_LOWER_CASE_STRING_IMPL_H_
+#define QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUICHE_LOWER_CASE_STRING_IMPL_H_
+
+#include <string>
+
+#include "absl/strings/ascii.h"
+#include "absl/strings/string_view.h"
+#include "quiche/common/platform/api/quiche_export.h"
+
+namespace quiche {
+
+class QUICHE_EXPORT_PRIVATE QuicheLowerCaseStringImpl {
+ public:
+  QuicheLowerCaseStringImpl(absl::string_view str)
+      : str_(absl::AsciiStrToLower(str)) {}
+
+  const std::string& get() const { return str_; }
+
+ private:
+  std::string str_;
+};
+
+}  // namespace quiche
+
+#endif  // QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUICHE_LOWER_CASE_STRING_IMPL_H_