Provide default implementation for QUIC protocol flags.

This is a subset of Chrome implementation; Chrome implementation also provides  SetQuicFlagByName(), which is only used in Chrome (it is not defined in google3).

PiperOrigin-RevId: 428784597

common/platform/default/quiche_platform_impl/quic_flags_impl.cc

diff --git a/common/platform/default/quiche_platform_impl/quic_flags_impl.cc b/common/platform/default/quiche_platform_impl/quic_flags_impl.cc
new file mode 100644
index 0000000..81b1d8f
--- /dev/null
+++ b/common/platform/default/quiche_platform_impl/quic_flags_impl.cc
@@ -0,0 +1,29 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "quiche_platform_impl/quic_flags_impl.h"
+
+#define DEFINE_QUIC_PROTOCOL_FLAG_SINGLE_VALUE(type, flag, value, doc) \
+  type FLAGS_##flag = value;
+
+#define DEFINE_QUIC_PROTOCOL_FLAG_TWO_VALUES(type, flag, internal_value, \
+                                             external_value, doc)        \
+  type FLAGS_##flag = external_value;
+
+// Preprocessor macros can only have one definition.
+// Select the right macro based on the number of arguments.
+#define GET_6TH_ARG(arg1, arg2, arg3, arg4, arg5, arg6, ...) arg6
+#define QUIC_PROTOCOL_FLAG_MACRO_CHOOSER(...)                    \
+  GET_6TH_ARG(__VA_ARGS__, DEFINE_QUIC_PROTOCOL_FLAG_TWO_VALUES, \
+              DEFINE_QUIC_PROTOCOL_FLAG_SINGLE_VALUE)
+#define QUIC_PROTOCOL_FLAG(...) \
+  QUIC_PROTOCOL_FLAG_MACRO_CHOOSER(__VA_ARGS__)(__VA_ARGS__)
+
+#include "quic/core/quic_protocol_flags_list.h"
+
+#undef QUIC_PROTOCOL_FLAG
+#undef QUIC_PROTOCOL_FLAG_MACRO_CHOOSER
+#undef GET_6TH_ARG
+#undef DEFINE_QUIC_PROTOCOL_FLAG_TWO_VALUES
+#undef DEFINE_QUIC_PROTOCOL_FLAG_SINGLE_VALUE

common/platform/default/quiche_platform_impl/quic_flags_impl.h

diff --git a/common/platform/default/quiche_platform_impl/quic_flags_impl.h b/common/platform/default/quiche_platform_impl/quic_flags_impl.h
new file mode 100644
index 0000000..b58d24d
--- /dev/null
+++ b/common/platform/default/quiche_platform_impl/quic_flags_impl.h
@@ -0,0 +1,18 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUIC_FLAGS_IMPL_H_
+#define QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUIC_FLAGS_IMPL_H_
+
+#include <string>
+
+#include "common/platform/api/quiche_export.h"
+#include "common/platform/api/quiche_flags.h"
+
+#define QUIC_PROTOCOL_FLAG(type, flag, ...) \
+  QUICHE_EXPORT_PRIVATE extern type FLAGS_##flag;
+#include "quic/core/quic_protocol_flags_list.h"
+#undef QUIC_PROTOCOL_FLAG
+
+#endif  // QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUIC_FLAGS_IMPL_H_

quic/platform/api/quic_flags.h

diff --git a/quic/platform/api/quic_flags.h b/quic/platform/api/quic_flags.h
index 43f81d4..762e590 100644
--- a/quic/platform/api/quic_flags.h
+++ b/quic/platform/api/quic_flags.h
@@ -8,8 +8,7 @@
 #include <string>
 #include <vector>
 
-#include "net/quic/platform/impl/quic_flags_impl.h"
-
+#include "quiche_platform_impl/quic_flags_impl.h"
 #include "common/platform/api/quiche_flags.h"
 
 #define GetQuicReloadableFlag(flag) GetQuicheReloadableFlag(quic, flag)