Ensure that the SingleFrameCodec stays alive until the ImageDecoder invokes its callback (#10297)

SingleFrameCodec had been holding a persistent handle to the peer Dart object
while waiting for the decoder callback.  However, Dart code could call
Codec.dispose during that interval, which would cause the Dart peer to drop
its reference to the SingleFrameCodec and potentially delete the
SingleFrameCodec.

Fixes https://github.com/flutter/flutter/issues/36631

lib/ui/painting/single_frame_codec.cc

@@ -50,18 +50,18 @@ Dart_Handle SingleFrameCodec::getNextFrame(Dart_Handle callback_handle) {
     return tonic::ToDart("Image decoder not available.");
   }
 
-  auto raw_codec_wrapper = new DartPersistentValue(
-      dart_state, Dart_HandleFromWeakPersistent(dart_wrapper()));
+  // The SingleFrameCodec must be deleted on the UI thread.  Allocate a RefPtr
+  // on the heap to ensure that the SingleFrameCodec remains alive until the
+  // decoder callback is invoked on the UI thread.  The callback can then
+  // drop the reference.
+  fml::RefPtr<SingleFrameCodec>* raw_codec_ref =
+      new fml::RefPtr<SingleFrameCodec>(this);
 
-  // We dont want to to put the raw codec in a lambda capture because we have
-  // to mutate (i.e destroy) it in the callback. Using MakeCopyable will create
-  // a shared pointer for the captures which can be destroyed on any thread. But
-  // we have to ensure that the DartPersistentValue is only destroyed on the UI
-  // thread.
-  decoder->Decode(descriptor_, [raw_codec_wrapper](auto image) {
-    std::unique_ptr<DartPersistentValue> codec_wrapper(raw_codec_wrapper);
+  decoder->Decode(descriptor_, [raw_codec_ref](auto image) {
+    std::unique_ptr<fml::RefPtr<SingleFrameCodec>> codec_ref(raw_codec_ref);
+    fml::RefPtr<SingleFrameCodec> codec(std::move(*codec_ref));
 
-    auto state = codec_wrapper->dart_state().lock();
+    auto state = codec->pending_callbacks_.front().dart_state().lock();
 
     if (!state) {
       // This is probably because the isolate has been terminated before the
@@ -72,9 +72,6 @@ Dart_Handle SingleFrameCodec::getNextFrame(Dart_Handle callback_handle) {
 
     tonic::DartState::Scope scope(state.get());
 
-    SingleFrameCodec* codec = tonic::DartConverter<SingleFrameCodec*>::FromDart(
-        codec_wrapper->value());
-
     if (image.get()) {
       auto canvas_image = fml::MakeRefCounted<CanvasImage>();
       canvas_image->set_image(std::move(image));