Mark used pictures so they aren't deleted on the next frame. (#19913)

lib/web_ui/lib/src/engine/compositor/canvas.dart

@@ -169,7 +169,7 @@ class CkCanvas {
   }
 
   void drawPicture(CkPicture picture) {
-    skCanvas.drawPicture(picture._skPicture);
+    skCanvas.drawPicture(picture.skiaObject.skiaObject);
   }
 
   void drawPoints(CkPaint paint, ui.PointMode pointMode,

lib/web_ui/lib/src/engine/compositor/picture.dart

@@ -5,13 +5,11 @@
 part of engine;
 
 class CkPicture implements ui.Picture {
-  final SkPicture _skPicture;
-  final SkiaObject skiaObject;
+  final SkiaObject<SkPicture> skiaObject;
   final ui.Rect? cullRect;
 
   CkPicture(SkPicture picture, this.cullRect)
-    : _skPicture = picture,
-      skiaObject = SkPictureSkiaObject(picture);
+      : skiaObject = SkPictureSkiaObject(picture);
 
   @override
   int get approximateBytesUsed => 0;
@@ -23,7 +21,8 @@ class CkPicture implements ui.Picture {
 
   @override
   Future<ui.Image> toImage(int width, int height) {
-    throw UnsupportedError('Picture.toImage not yet implemented for CanvasKit and HTML');
+    throw UnsupportedError(
+        'Picture.toImage not yet implemented for CanvasKit and HTML');
   }
 }