BoxLayout: Added set_collapse_when_hidden(), like views::Label.

ui/views/layout/box_layout.h

Index: ui/views/layout/box_layout.h
diff --git a/ui/views/layout/box_layout.h b/ui/views/layout/box_layout.h
index 0a82bb4322c831d09917a94455a33ed2f1717c6a..e3b8ea5b0a7c39c04751f2a4dd61c59299d848da 100644
--- a/ui/views/layout/box_layout.h
+++ b/ui/views/layout/box_layout.h
@@ -81,6 +81,9 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
     minimum_cross_axis_size_ = size;
   }
 
+  // Sets whether the preferred size is empty when the host is not visible.
+  void set_collapse_when_hidden(bool value) { collapse_when_hidden_ = value; }
+
   // Sets the flex weight for the given |view|. Using the preferred size as
   // the basis, free space along the main axis is distributed to views in the
   // ratio of their flex weights. Similarly, if the views will overflow the
@@ -165,6 +168,9 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   // The minimum cross axis size for the layout.
   int minimum_cross_axis_size_;
 
+  // Whether to collapse the host when it's not visible.
+  bool collapse_when_hidden_;
+
   // The view that this BoxLayout is managing the layout for.
   views::View* host_;