WorkspaceLayoutManager backdrop should fill entire region.

ash/wm/workspace/workspace_layout_manager_delegate.h

Index: ash/wm/workspace/workspace_layout_manager_delegate.h
diff --git a/ash/wm/workspace/workspace_layout_manager_delegate.h b/ash/wm/workspace/workspace_layout_manager_delegate.h
index 4cd17108852388d06336359a7089c20f4cf870e3..cdc76c21eca8d3c7960a7ba7f7b5936dc4b6840b 100644
--- a/ash/wm/workspace/workspace_layout_manager_delegate.h
+++ b/ash/wm/workspace/workspace_layout_manager_delegate.h
@@ -39,6 +39,9 @@ class WorkspaceLayoutManagerDelegate {
   // A window state type has changed.
   virtual void OnPostWindowStateTypeChange(wm::WindowState* window_state,
                                            wm::WindowStateType old_type) = 0;
+
+  // The work area insets have changed, altering the total available space.
+  virtual void OnDisplayWorkAreaInsetsChanged() = 0;
 };
 
 }  // namespace ash