Use anti-aliasing when drawing text in the performance overlay (flutter/engine#7445)

The engine dropped the ability to draw non anti-aliased text in
a10b062df0

Fixes https://github.com/flutter/flutter/issues/26387

engine/src/flutter/flow/layers/performance_overlay_layer.cc

@@ -19,6 +19,7 @@ void DrawStatisticsText(SkCanvas& canvas,
   paint.setTextSize(15);
   paint.setLinearText(false);
   paint.setColor(SK_ColorGRAY);
+  paint.setAntiAlias(true);
   canvas.drawText(string.c_str(), string.size(), x, y, paint);
 }