Double clicking the resize handle should expand window along that axis.

ui/aura/test/test_window_delegate.h

Index: ui/aura/test/test_window_delegate.h
===================================================================
--- ui/aura/test/test_window_delegate.h	(revision 124527)
+++ ui/aura/test/test_window_delegate.h	(working copy)
@@ -19,6 +19,10 @@
   TestWindowDelegate();
   virtual ~TestWindowDelegate();
 
+  void set_window_component(int window_component) {
+    window_component_ = window_component;
+  }
+
   // Overridden from WindowDelegate:
   virtual gfx::Size GetMinimumSize() const OVERRIDE;
   virtual void OnBoundsChanged(const gfx::Rect& old_bounds,
@@ -39,6 +43,8 @@
   virtual void OnWindowVisibilityChanged(bool visible) OVERRIDE;
 
  private:
+  int window_component_;
+
   DISALLOW_COPY_AND_ASSIGN(TestWindowDelegate);
 };