Desktop aura: Break aura::Window::SetParent in two.

ui/aura/window.h

Index: ui/aura/window.h
diff --git a/ui/aura/window.h b/ui/aura/window.h
index ccfdc4810d599015a1f0280a9429b8fe033a984b..60aeba34fb28d9414c9d644ace2993c895760167 100644
--- a/ui/aura/window.h
+++ b/ui/aura/window.h
@@ -171,9 +171,16 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
   // Assigns a new external texture to the window's layer.
   void SetExternalTexture(ui::Texture* texture);
 
-  // Sets the parent window of the window. If NULL, the window is parented to
-  // the root window.
-  void SetParent(Window* parent);
+  // Sets the parent window of the window. Can not be NULL.

[Ben Goodger (Google), 2012/11/20 23:46:38]

Cannot or Must not

+  void SetParentTo(Window* parent);
+
+  // Attach this window somewhere.
+  //
+  // |context_root| can never be NULL; since desktop aura and ash can be run at

[Ben Goodger (Google), 2012/11/20 23:46:38]

Don't leak knowledge of downstream components into an upstream comment.

// |target_root| must not be NULL.
void SetDefaultParentByTargetRoot(RootWindow* target_root);

+  // the same time in the same process, we always must have some context to
+  // determine desktop vs ash semantics. Users shouldn't assume that the window
+  // will be rooted to |context_root|.
+  void SetDefaultParentByTargetRoot(RootWindow* context_root);
 
   // Stacks the specified child of this Window at the front of the z-order.
   void StackChildAtTop(Window* child);