Improve CrOS WM shadow drawing for small windows.

ui/wm/core/shadow.h

Index: ui/wm/core/shadow.h
diff --git a/ui/wm/core/shadow.h b/ui/wm/core/shadow.h
index bc7053f104e44186ea59bfae7b47cd8ec64c5de7..8117c467808d057cf091e1efed6a2179ef2811fd 100644
--- a/ui/wm/core/shadow.h
+++ b/ui/wm/core/shadow.h
@@ -92,6 +92,9 @@ class WM_EXPORT Shadow : public ui::ImplicitAnimationObserver {
   // Bounds of the content that the shadow encloses.
   gfx::Rect content_bounds_;
 
+  // The elevation of the shadow image that's currently set on |shadow_layer_|.
+  int effective_elevation_ = 0;
+
   DISALLOW_COPY_AND_ASSIGN(Shadow);
 };