Add ui::LayerObserver and use it to update Alt+Tab previews as needed.

ui/wm/core/window_util.cc

Index: ui/wm/core/window_util.cc
diff --git a/ui/wm/core/window_util.cc b/ui/wm/core/window_util.cc
index 07723511c841a3fd3c58726520c205064cc1c062..bdfaee5981c440c4c335dd000f6ec044b9393cef 100644
--- a/ui/wm/core/window_util.cc
+++ b/ui/wm/core/window_util.cc
@@ -28,7 +28,7 @@ void CloneChildren(ui::Layer* to_clone,
     if (old_layer) {
       if (factory && owner->layer()->delegate())
         old_layer->set_delegate(
-            factory->CreateDelegate(owner->layer()->delegate()));
+            factory->CreateDelegate(old_layer, owner->layer()));
       parent->Add(old_layer);
       // RecreateLayer() moves the existing children to the new layer. Create a
       // copy of those.
@@ -91,10 +91,9 @@ std::unique_ptr<ui::LayerTreeOwner> RecreateLayers(
   std::unique_ptr<ui::LayerTreeOwner> old_layer(
       new ui::LayerTreeOwner(root->RecreateLayer().release()));
   if (old_layer->root()) {
-    if (factory) {

[sky, 2016/08/23 17:15:07]

optional: personally I would leave the {} here as the body spans multiple lines.
But I realize the style guide is unclear on this.

[Evan Stade, 2016/08/29 23:51:54]

oops, this was not intentional

+    if (factory)
       old_layer->root()->set_delegate(
-          factory->CreateDelegate(root->layer()->delegate()));
-    }
+          factory->CreateDelegate(old_layer->root(), root->layer()));
     CloneChildren(root->layer(), old_layer->root(), factory);
   }
   return old_layer;