Add a NOT_DRAWN window in between the root_window and its children.

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 6ba028a3fd122aa41e534c4abf0d679e01904e21..e0bdf29a00849a274d7060e9d07cbd7e1d9aebfb 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -868,37 +868,46 @@ void RootWindowController::InitLayoutManagers() {
 
 void RootWindowController::CreateContainers() {
   WmWindow* root = GetWindow();
+  // For screen rotation animation: add a NOT_DRAWN layer in between the
+  // root_window's layer and its current children so that we only need to
+  // initiate two LayerAnimationSequences. One for the new layers and one for
+  // the old layers.
+  WmWindow* delegate_root = CreateContainer(
+      kShellWindowId_DelegateRootContainer, "DelegateRootContainer", root);

[oshima, 2017/04/05 20:26:51]

"delegate" is overused. maybe surrogate root? 

+jamescook@ who probably have a better suggestion.

[James Cook, 2017/04/05 20:54:41]

Why not just ScreenRotationContainer, since that's all it's used for?

[wutao, 2017/04/05 23:37:34]

Done.

+  delegate_root->SetChildWindowVisibilityChangesAnimated();
+  delegate_root->GetLayer()->set_name("DelegateRootLayer");
+
   // These containers are just used by PowerButtonController to animate groups
   // of containers simultaneously without messing up the current transformations
-  // on those containers. These are direct children of the root window; all of
-  // the other containers are their children.
+  // on those containers. These are direct children of the delegate_root window;
+  // all of the other containers are their children.
 
   // The wallpaper container is not part of the lock animation, so it is not
   // included in those animate groups. When the screen is locked, the wallpaper
   // is moved to the lock screen wallpaper container (and moved back on unlock).
   // Ensure that there's an opaque layer occluding the non-lock-screen layers.
   WmWindow* wallpaper_container = CreateContainer(
-      kShellWindowId_WallpaperContainer, "WallpaperContainer", root);
+      kShellWindowId_WallpaperContainer, "WallpaperContainer", delegate_root);
   wallpaper_container->SetChildWindowVisibilityChangesAnimated();
 
   WmWindow* non_lock_screen_containers =
       CreateContainer(kShellWindowId_NonLockScreenContainersContainer,
-                      "NonLockScreenContainersContainer", root);
+                      "NonLockScreenContainersContainer", delegate_root);
   // Clip all windows inside this container, as half pixel of the window's
   // texture may become visible when the screen is scaled. crbug.com/368591.
   non_lock_screen_containers->SetMasksToBounds(true);
 
   WmWindow* lock_wallpaper_containers =
       CreateContainer(kShellWindowId_LockScreenWallpaperContainer,
-                      "LockScreenWallpaperContainer", root);
+                      "LockScreenWallpaperContainer", delegate_root);
   lock_wallpaper_containers->SetChildWindowVisibilityChangesAnimated();
 
   WmWindow* lock_screen_containers =
       CreateContainer(kShellWindowId_LockScreenContainersContainer,
-                      "LockScreenContainersContainer", root);
+                      "LockScreenContainersContainer", delegate_root);
   WmWindow* lock_screen_related_containers =
       CreateContainer(kShellWindowId_LockScreenRelatedContainersContainer,
-                      "LockScreenRelatedContainersContainer", root);
+                      "LockScreenRelatedContainersContainer", delegate_root);
 
   CreateContainer(kShellWindowId_UnparentedControlContainer,
                   "UnparentedControlContainer", non_lock_screen_containers);
@@ -1025,13 +1034,14 @@ void RootWindowController::CreateContainers() {
   overlay_container->SetBoundsInScreenBehaviorForChildren(
       WmWindow::BoundsInScreenBehavior::USE_SCREEN_COORDINATES);
 
-  WmWindow* mouse_cursor_container = CreateContainer(
-      kShellWindowId_MouseCursorContainer, "MouseCursorContainer", root);
+  WmWindow* mouse_cursor_container =
+      CreateContainer(kShellWindowId_MouseCursorContainer,
+                      "MouseCursorContainer", delegate_root);
   mouse_cursor_container->SetBoundsInScreenBehaviorForChildren(
       WmWindow::BoundsInScreenBehavior::USE_SCREEN_COORDINATES);
 
   CreateContainer(kShellWindowId_PowerButtonAnimationContainer,
-                  "PowerButtonAnimationContainer", root);
+                  "PowerButtonAnimationContainer", delegate_root);
 }
 
 void RootWindowController::CreateSystemWallpaper(