Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/wm/workspace/workspace_layout_manager.cc

Index: ash/wm/workspace/workspace_layout_manager.cc
diff --git a/ash/wm/workspace/workspace_layout_manager.cc b/ash/wm/workspace/workspace_layout_manager.cc
index 716907d266aab265355f96ac0468e69556fa03c7..5ae728bb14fb0e8eea4036bd451616be7e9c8cee 100644
--- a/ash/wm/workspace/workspace_layout_manager.cc
+++ b/ash/wm/workspace/workspace_layout_manager.cc
@@ -60,6 +60,11 @@ void WorkspaceLayoutManager::SetShelf(internal::ShelfLayoutManager* shelf) {
   shelf_ = shelf;
 }
 
+void WorkspaceLayoutManager::SetMaximizeBackdropDelegate(
+    MaximizeBackdropDelegate* delegate) {
+  backdrop_delegate_.reset(delegate);
+}
+
 //////////////////////////////////////////////////////////////////////////////
 // WorkspaceLayoutManager, aura::LayoutManager implementation:
 
@@ -72,6 +77,8 @@ void WorkspaceLayoutManager::OnWindowAddedToLayout(Window* child) {
   window_state->AddObserver(this);
   UpdateShelfVisibility();
   UpdateFullscreenState();
+  if (backdrop_delegate_)
+    backdrop_delegate_->RestackBackdrop();
   WindowPositioner::RearrangeVisibleWindowOnShow(child);
 }
 
@@ -87,6 +94,8 @@ void WorkspaceLayoutManager::OnWillRemoveWindowFromLayout(Window* child) {
 void WorkspaceLayoutManager::OnWindowRemovedFromLayout(Window* child) {
   UpdateShelfVisibility();
   UpdateFullscreenState();
+  if (backdrop_delegate_)
+    backdrop_delegate_->RestackBackdrop();
 }
 
 void WorkspaceLayoutManager::OnChildWindowVisibilityChanged(Window* child,
@@ -102,6 +111,8 @@ void WorkspaceLayoutManager::OnChildWindowVisibilityChanged(Window* child,
     WindowPositioner::RearrangeVisibleWindowOnHideOrRemove(child);
   UpdateFullscreenState();
   UpdateShelfVisibility();
+  if (backdrop_delegate_)
+    backdrop_delegate_->RestackBackdrop();
 }
 
 void WorkspaceLayoutManager::SetChildBounds(
@@ -162,6 +173,8 @@ void WorkspaceLayoutManager::OnWindowPropertyChanged(Window* window,
 void WorkspaceLayoutManager::OnWindowStackingChanged(aura::Window* window) {
   UpdateShelfVisibility();
   UpdateFullscreenState();
+  if (backdrop_delegate_)
+    backdrop_delegate_->RestackBackdrop();
 }
 
 void WorkspaceLayoutManager::OnWindowDestroying(aura::Window* window) {
@@ -210,6 +223,8 @@ void WorkspaceLayoutManager::OnPostWindowStateTypeChange(
   }
 
   UpdateShelfVisibility();
+  if (backdrop_delegate_)
+    backdrop_delegate_->RestackBackdrop();
 }
 
 //////////////////////////////////////////////////////////////////////////////