Make text blob struct.

Runtime/ui/painting/canvas_impl.cs

         }
 
         void _drawTextBlob(TextBlob textBlob, Offset offset, Paint paint) {
-            D.assert(textBlob != null);
             D.assert(offset != null);
             D.assert(paint != null);
 

Runtime/ui/painting/txt/text_blob.cs

 namespace Unity.UIWidgets.ui {
-    public class TextBlob {
+    public struct TextBlob {
         internal TextBlob(string text, int textOffset, int textSize, Vector2d[] positions, Rect bounds,
             TextStyle style) {
             this.instanceId = ++_nextInstanceId;