Compute if a layer is drawn without LayerTree hierarchy

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 143a4e7d08c2b1121881788e22a40184cd91e4bf..45efd2a22b8d7cd89cd1dfb5d5a4db5c9cb999dc 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -514,6 +514,10 @@ class COMPOSITOR_EXPORT Layer
 
   // A cached copy of |Compositor::device_scale_factor()|.
   float device_scale_factor_;
+  // When a layer become invisible, we make the corresponding cc_layer
+  // transparent (opacity = 0) and cache its actual opacity value. This is used
+  // to set it back when the layer becomes visible.
+  float cached_cc_layer_opacity_;
 
   // A cached copy of the nine patch layer's image and aperture.
   // These are required for device scale factor change.