compositor: Add protected virtual destructor to CompositorDelegate.

compositor: Add protected virtual destructor to CompositorDelegate.

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

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

Example:

class Delegate {
 public:
  ...

 protected:
  virtual ~Delegate() {}
};

class Foo : public Delegate {
};

Delegate* delegate = new Foo;
delete delegate;  // It should prevent this situation!

R=sky@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=102350

[sky, 2011-09-22 17:44:15]

LGTM