Add FlJsonMessageCodec

Author: robert-ancell

ci/licenses_golden/licenses_flutter

@@ -1193,6 +1193,8 @@ FILE: ../../../flutter/shell/platform/linux/fl_dart_project.cc
 FILE: ../../../flutter/shell/platform/linux/fl_dart_project_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine_private.h
+FILE: ../../../flutter/shell/platform/linux/fl_json_message_codec.cc
+FILE: ../../../flutter/shell/platform/linux/fl_json_message_codec_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_message_codec.cc
 FILE: ../../../flutter/shell/platform/linux/fl_message_codec_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_method_channel.cc
@@ -1219,6 +1221,7 @@ FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_binary_codec
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_binary_messenger.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_dart_project.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_engine.h
+FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_json_message_codec.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_message_codec.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_method_channel.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_method_codec.h

shell/platform/linux/BUILD.gn

@@ -48,6 +48,7 @@ _public_headers = [
   "public/flutter_linux/fl_binary_messenger.h",
   "public/flutter_linux/fl_dart_project.h",
   "public/flutter_linux/fl_engine.h",
+  "public/flutter_linux/fl_json_message_codec.h",
   "public/flutter_linux/fl_message_codec.h",
   "public/flutter_linux/fl_method_channel.h",
   "public/flutter_linux/fl_method_codec.h",
@@ -72,6 +73,7 @@ source_set("flutter_linux") {
     "fl_binary_messenger.cc",
     "fl_dart_project.cc",
     "fl_engine.cc",
+    "fl_json_message_codec.cc",
     "fl_message_codec.cc",
     "fl_method_channel.cc",
     "fl_method_codec.cc",
@@ -109,6 +111,7 @@ executable("flutter_linux_unittests") {
   sources = [
     "fl_binary_codec_test.cc",
     "fl_dart_project_test.cc",
+    "fl_json_message_codec_test.cc",
     "fl_message_codec_test.cc",
     "fl_method_codec_test.cc",
     "fl_method_response_test.cc",