Replace QuicheOptional with absl::optional.

PiperOrigin-RevId: 336947387
Change-Id: I20fa90b62347b2fbb08e68db34a22cd182ef2b64
diff --git a/http2/test_tools/frame_parts.cc b/http2/test_tools/frame_parts.cc
index 95ea34d..bb975e2 100644
--- a/http2/test_tools/frame_parts.cc
+++ b/http2/test_tools/frame_parts.cc
@@ -506,10 +506,9 @@
   return AssertionSuccess();
 }
 
-AssertionResult FrameParts::AppendString(
-    absl::string_view source,
-    std::string* target,
-    quiche::QuicheOptional<size_t>* opt_length) {
+AssertionResult FrameParts::AppendString(absl::string_view source,
+                                         std::string* target,
+                                         absl::optional<size_t>* opt_length) {
   target->append(source.data(), source.size());
   if (opt_length != nullptr) {
     VERIFY_TRUE(*opt_length) << "Length is not set yet\n" << *this;