[web] Correctly read the repetitionCount for images (#162414)

There was a bug in `ResizingCodec` where it returned `frameCount`
instead of `repetitionCount` in the `repetitionCount` getter, causing
animated images to stop animating even if they were set to loop
infinitely.

Fixes https://github.com/flutter/flutter/issues/161800

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engine/src/flutter/lib/web_ui/lib/src/engine/image_decoder.dart

@@ -244,7 +244,7 @@ class ResizingCodec implements ui.Codec {
   );
 
   @override
-  int get repetitionCount => delegate.frameCount;
+  int get repetitionCount => delegate.repetitionCount;
 }
 
 BitmapSize? scaledImageSize(int width, int height, int? targetWidth, int? targetHeight) {

engine/src/flutter/lib/web_ui/test/ui/image_decoder_test.dart

@@ -0,0 +1,26 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+
+import '../common/test_initialization.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+Future<void> testMain() async {
+  setUpUnitTests(emulateTesterEnvironment: false, setUpTestViewDimensions: false);
+
+  test('$ResizingCodec gives correct repetition count for GIFs', () async {
+    final ui.Codec codec = await renderer.instantiateImageCodecFromUrl(
+      Uri(path: '/test_images/required.gif'),
+    );
+    final ui.Codec resizingCodec = ResizingCodec(codec);
+    expect(resizingCodec.repetitionCount, 0);
+  });
+}