Migrate PlatformMessageHandlerIos to ARC (flutter#52226)

Smart pointers support ARC as of flutter/engine#47612, and the unit tests were migrated in flutter/engine#48162.

Migrate `PlatformMessageHandlerIos` from MRC to ARC.  Clean up the `#include`s.

Part of flutter#137801.

Author: jmagman

shell/platform/darwin/ios/BUILD.gn

@@ -117,6 +117,8 @@ source_set("flutter_framework_source_arc") {
     "ios_surface_metal_skia.mm",
     "ios_surface_software.h",
     "ios_surface_software.mm",
+    "platform_message_handler_ios.h",
+    "platform_message_handler_ios.mm",
     "rendering_api_selection.h",
     "rendering_api_selection.mm",
   ]
@@ -180,8 +182,6 @@ source_set("flutter_framework_source") {
     "framework/Source/accessibility_text_entry.mm",
     "ios_external_view_embedder.h",
     "ios_external_view_embedder.mm",
-    "platform_message_handler_ios.h",
-    "platform_message_handler_ios.mm",
     "platform_view_ios.h",
     "platform_view_ios.mm",
   ]

shell/platform/darwin/ios/platform_message_handler_ios.h

@@ -5,13 +5,10 @@
 #ifndef FLUTTER_SHELL_PLATFORM_DARWIN_IOS_PLATFORM_MESSAGE_HANDLER_IOS_H_
 #define FLUTTER_SHELL_PLATFORM_DARWIN_IOS_PLATFORM_MESSAGE_HANDLER_IOS_H_
 
-#include <unordered_map>
-
-#include "flutter/common/task_runners.h"
 #include "flutter/fml/platform/darwin/scoped_block.h"
 #include "flutter/fml/platform/darwin/scoped_nsobject.h"
+#include "flutter/fml/task_runner.h"
 #include "flutter/shell/common/platform_message_handler.h"
-#import "flutter/shell/platform/darwin/common/framework/Headers/FlutterBinaryMessenger.h"
 #import "flutter/shell/platform/darwin/ios/flutter_task_queue_dispatch.h"
 
 namespace flutter {

shell/platform/darwin/ios/platform_message_handler_ios.mm

@@ -4,15 +4,17 @@
 
 #import "flutter/shell/platform/darwin/ios/platform_message_handler_ios.h"
 
-#import "flutter/fml/trace_event.h"
-#import "flutter/lib/ui/window/platform_message.h"
-#import "flutter/shell/platform/darwin/common/buffer_conversions.h"
-#import "flutter/shell/platform/darwin/common/framework/Headers/FlutterBinaryMessenger.h"
+#include "flutter/fml/trace_event.h"
+#include "flutter/lib/ui/window/platform_message.h"
+#include "flutter/lib/ui/window/platform_message_response.h"
+#include "flutter/shell/platform/darwin/common/buffer_conversions.h"
+
+FLUTTER_ASSERT_ARC
 
 static uint64_t platform_message_counter = 1;
 
 @interface FLTSerialTaskQueue : NSObject <FlutterTaskQueueDispatch>
-@property(nonatomic, strong) dispatch_queue_t queue;
+@property(nonatomic, readonly) dispatch_queue_t queue;
 @end
 
 @implementation FLTSerialTaskQueue
@@ -24,11 +26,6 @@ - (instancetype)init {
   return self;
 }
 
-- (void)dealloc {
-  dispatch_release(_queue);
-  [super dealloc];
-}
-
 - (void)dispatch:(dispatch_block_t)block {
   dispatch_async(self.queue, block);
 }
@@ -37,7 +34,7 @@ - (void)dispatch:(dispatch_block_t)block {
 namespace flutter {
 
 NSObject<FlutterTaskQueue>* PlatformMessageHandlerIos::MakeBackgroundTaskQueue() {
-  return [[[FLTSerialTaskQueue alloc] init] autorelease];
+  return [[FLTSerialTaskQueue alloc] init];
 }
 
 PlatformMessageHandlerIos::PlatformMessageHandlerIos(
@@ -127,8 +124,8 @@ - (void)dispatch:(dispatch_block_t)block {
   message_handlers_.erase(channel);
   if (handler) {
     message_handlers_[channel] = {
-        .task_queue = fml::scoped_nsprotocol(
-            [static_cast<NSObject<FlutterTaskQueueDispatch>*>(task_queue) retain]),
+        .task_queue =
+            fml::scoped_nsprotocol(static_cast<NSObject<FlutterTaskQueueDispatch>*>(task_queue)),
         .handler =
             fml::ScopedBlock<FlutterBinaryMessageHandler>{
                 handler, fml::scoped_policy::OwnershipPolicy::kRetain},