iOS/macOS: migrate darwin/common to ARC (flutter/engine#56155)

Migrates //flutter/shell/platform/darwin/common` targets to ARC. Most of
these were already ARC compatible.

This does add use of the `FLUTTER_ASSERT_ARC` define since that is
defined in a public framework header, but this code has no dependency on
the framework.

This removes `flutter_cflags_objc` and `flutter_cflags_objcc` compile
configs from `availability_version_check_unittests` since that target is
pure C++ and contains no Objective-C code.

No test changes since this there is no semantic change to the code.

Issue: flutter#137801

Author: cbracken

engine/src/flutter/shell/platform/darwin/common/BUILD.gn

@@ -9,8 +9,8 @@ import("//flutter/testing/testing.gni")
 import("framework_common.gni")
 
 source_set("common") {
-  cflags_objc = flutter_cflags_objc
-  cflags_objcc = flutter_cflags_objcc
+  cflags_objc = flutter_cflags_objc_arc
+  cflags_objcc = flutter_cflags_objcc_arc
 
   sources = [
     "buffer_conversions.h",
@@ -40,9 +40,6 @@ source_set("common") {
 # See: Upstream clang change: https://reviews.llvm.org/D150397
 # See: https://github.com/flutter/flutter/issues/133777
 source_set("availability_version_check") {
-  cflags_objc = flutter_cflags_objc
-  cflags_objcc = flutter_cflags_objcc
-
   sources = [ "availability_version_check.cc" ]
 
   deps = [ "//flutter/fml" ]

engine/src/flutter/shell/platform/darwin/common/buffer_conversions.mm

@@ -7,22 +7,22 @@
 #include "flutter/fml/macros.h"
 #include "flutter/fml/platform/darwin/scoped_nsobject.h"
 
+static_assert(__has_feature(objc_arc), "ARC must be enabled.");
+
 namespace flutter {
 namespace {
 class NSDataMapping : public fml::Mapping {
  public:
-  explicit NSDataMapping(NSData* data) : data_([data retain]) {}
+  explicit NSDataMapping(NSData* data) : data_(data) {}
 
-  size_t GetSize() const override { return [data_.get() length]; }
+  size_t GetSize() const override { return data_.length; }
 
-  const uint8_t* GetMapping() const override {
-    return static_cast<const uint8_t*>([data_.get() bytes]);
-  }
+  const uint8_t* GetMapping() const override { return static_cast<const uint8_t*>(data_.bytes); }
 
   bool IsDontNeedSafe() const override { return false; }
 
  private:
-  fml::scoped_nsobject<NSData> data_;
+  NSData* data_;
   FML_DISALLOW_COPY_AND_ASSIGN(NSDataMapping);
 };
 }  // namespace

engine/src/flutter/shell/platform/darwin/common/command_line.mm

@@ -6,6 +6,8 @@
 
 #import <Foundation/Foundation.h>
 
+static_assert(__has_feature(objc_arc), "ARC must be enabled.");
+
 namespace flutter {
 
 fml::CommandLine CommandLineFromNSProcessInfo(NSProcessInfo* processInfoOrNil) {