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

ui/compositor/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
 component("compositor") {
   sources = [
     "callback_layer_animation_observer.cc",
@@ skipping 28 matching lines @@
     "layer_animation_element.h",
     "layer_animation_observer.cc",
     "layer_animation_observer.h",
     "layer_animation_sequence.cc",
     "layer_animation_sequence.h",
     "layer_animator.cc",
     "layer_animator.h",
     "layer_animator_collection.cc",
     "layer_animator_collection.h",
     "layer_delegate.h",
+    "layer_observer.h",
     "layer_owner.cc",
     "layer_owner.h",
     "layer_threaded_animation_delegate.h",
     "layer_tree_owner.cc",
     "layer_tree_owner.h",
     "layer_type.h",
     "paint_cache.cc",
     "paint_cache.h",
     "paint_context.cc",
     "paint_context.h",
@@ skipping 137 matching lines @@
     "//ui/base",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl",
     "//ui/resources",
   ]
   if (is_linux) {
     deps += [ "//third_party/mesa:osmesa" ]
   }
 }