ui/base/models: Add protected virtual destructor to ComboboxModel.

ui/base/models: Add protected virtual destructor to ComboboxModel.

The use of a protected virtual destructor is to prevent the destruction of a
derived object via a base-class pointer.

That's it, ComboboxModel should only be deleted through derived class.

Example 1:

class FooComboboxModel : public ComboboxModel {
};

ComboboxModel* model = new FooComboboxModel;
delete model;  // It should prevent this situation!

Also, a protected virtual destructor makes it clear that someone else owns the
object.

Example 2:

class Foo {
 public:
  void SetModel(ComboboxModel* model);
};

If you see that the model has a protected destructor, you know Foo doesn't own
|model|.

R=sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=128132

[sky, 2012-03-21 16:28:23]

LGTM

[Evan Stade, 2012-11-28 03:51:08]

I don't understand the purpose of this change. It's useful to allow Example 1.
As in,

  ComboboxModel* model =
      boolean_expression ? new FooComboboxModel : new BarComboboxModel;
  // use model
  // ...
  delete model;

[sky, 2012-11-28 05:18:58]

We generally use this pattern to make it clear the consumer of the ComboboxModel
doesn't own it, since it can't delete it. You'll see this pattern used for
Observers too.

[Evan Stade, 2012-11-28 21:31:33]

On 2012/11/28 05:18:58, sky wrote:
> We generally use this pattern to make it clear the consumer of the
ComboboxModel
> doesn't own it, since it can't delete it. You'll see this pattern used for
> Observers too.

I do see that for some observers as well as some delegates, but sometimes the
owner does not know the type. For example, WebContentsViewAura owns a pointer of
type WebContentsViewDelegate. Do you think that this is a valid exception that
justifies WebContentsViewDelegate's public destructor, or that that usage should
be adjusted? If the former, what constitutes a valid justification? If the
latter, how could that be done gracefully?