Adds Window::MoveChildToFront, with surrounding changes.

ui/gfx/compositor/layer.h

Index: ui/gfx/compositor/layer.h
diff --git a/ui/gfx/compositor/layer.h b/ui/gfx/compositor/layer.h
index 9b279e0dc53e02e6f54ee95c781ef990da16736a..cde70e9f8054fb76f3fcd3c8610e5f99b2de0216 100644
--- a/ui/gfx/compositor/layer.h
+++ b/ui/gfx/compositor/layer.h
@@ -74,6 +74,10 @@ class COMPOSITOR_EXPORT Layer :
   // Moves a child to the end of the child list.
   void MoveToFront(Layer* child);
 
+  // Moves |child| to be above |other|. Does nothing if |other| is already above
+  // |child|.
+  void MoveAbove(Layer* child, Layer* other);
+
   // Returns the child Layers.
   const std::vector<Layer*>& children() const { return children_; }