Use .mm extension for FlutterStandardCodecHelper.c

Even though the file is pure C code, it's useful to use a C++ filename
in order to use FML (assertions) in the implementation.

No semantic changes so no test changes.

Author: cbracken

ci/licenses_golden/licenses_flutter

@@ -2417,7 +2417,7 @@ ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterCh
 ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterChannelsTest.m + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterCodecs.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodec.mm + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodecHelper.c + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodecHelper.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodecHelper.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodec_Internal.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/graphics/FlutterDarwinContextMetalImpeller.h + ../../../flutter/LICENSE
@@ -4874,7 +4874,7 @@ FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterChan
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterChannelsTest.m
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterCodecs.mm
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodec.mm
-FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodecHelper.c
+FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodecHelper.mm
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodecHelper.h
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodec_Internal.h
 FILE: ../../../flutter/shell/platform/darwin/graphics/FlutterDarwinContextMetalImpeller.h

shell/platform/darwin/BUILD.gn

@@ -47,7 +47,7 @@ source_set("flutter_channels_arc") {
     "common/framework/Source/FlutterChannels.mm",
     "common/framework/Source/FlutterCodecs.mm",
     "common/framework/Source/FlutterStandardCodec.mm",
-    "common/framework/Source/FlutterStandardCodecHelper.c",
+    "common/framework/Source/FlutterStandardCodecHelper.mm",
     "common/framework/Source/FlutterStandardCodec_Internal.h",
   ]
 

shell/platform/darwin/common/BUILD.gn

@@ -37,7 +37,7 @@ source_set("framework_shared") {
     "framework/Source/FlutterChannels.mm",
     "framework/Source/FlutterCodecs.mm",
     "framework/Source/FlutterStandardCodec.mm",
-    "framework/Source/FlutterStandardCodecHelper.c",
+    "framework/Source/FlutterStandardCodecHelper.mm",
     "framework/Source/FlutterStandardCodec_Internal.h",
   ]