Do not use std::string::append(absl::string_view) in QUICHE.

Even though absl::string_view is supported in Chromium (in third-party
libraries), this append() overload does not compile, preventing merge.  Use a
more traditional overload instead.

PiperOrigin-RevId: 351154388
Change-Id: Iaf57f9efbb5733fecb76261a1bfe64393d2b304a

http2/test_tools/frame_parts.cc

diff --git a/http2/test_tools/frame_parts.cc b/http2/test_tools/frame_parts.cc
index aea23a9..1a14870 100644
--- a/http2/test_tools/frame_parts.cc
+++ b/http2/test_tools/frame_parts.cc
@@ -382,7 +382,7 @@
 void FrameParts::OnPriorityUpdatePayload(const char* data, size_t len) {
   HTTP2_VLOG(1) << "OnPriorityUpdatePayload: len=" << len;
   ASSERT_TRUE(InFrameOfType(Http2FrameType::PRIORITY_UPDATE)) << *this;
-  payload_.append(absl::string_view(data, len));
+  payload_.append(data, len);
 }
 
 void FrameParts::OnPriorityUpdateEnd() {