Move quic_flags.h from net/quic/core to net/quic/platform and split into api and impl.

net/BUILD.gn

Index: net/BUILD.gn
diff --git a/net/BUILD.gn b/net/BUILD.gn
index 28477ee62f3afd246bdc466f4ea4346495454ef6..f9bb49fd9472ab4c2623fbe23c3dcab7b5e6ef63 100644
--- a/net/BUILD.gn
+++ b/net/BUILD.gn
@@ -1267,8 +1267,6 @@ component("net") {
       "quic/core/quic_data_writer.h",
       "quic/core/quic_error_codes.cc",
       "quic/core/quic_error_codes.h",
-      "quic/core/quic_flags.cc",
-      "quic/core/quic_flags.h",
       "quic/core/quic_flags_list.h",
       "quic/core/quic_flow_controller.cc",
       "quic/core/quic_flow_controller.h",
@@ -1342,6 +1340,7 @@ component("net") {
       "quic/platform/api/quic_endian.h",
       "quic/platform/api/quic_estimate_memory_usage.h",
       "quic/platform/api/quic_export.h",
+      "quic/platform/api/quic_flags.h",
       "quic/platform/api/quic_flag_utils.h",
       "quic/platform/api/quic_hostname_utils.cc",
       "quic/platform/api/quic_hostname_utils.h",
@@ -1373,6 +1372,8 @@ component("net") {
       "quic/platform/impl/quic_endian_impl.h",
       "quic/platform/impl/quic_estimate_memory_usage_impl.h",
       "quic/platform/impl/quic_export_impl.h",
+      "quic/platform/impl/quic_flags_impl.cc",
+      "quic/platform/impl/quic_flags_impl.h",
       "quic/platform/impl/quic_flag_utils_impl.h",
       "quic/platform/impl/quic_hostname_utils_impl.cc",
       "quic/platform/impl/quic_hostname_utils_impl.h",