We should make all inheritance from angle::NonCopyable private so the compiler will complain about this (admittedly unlikely) code:
    
      class Foo: angle::NonCopyable {
          virtual ~Foo() { ... }
      };
      angle::NonCopyable *p = new Foo;
      delete p;
    
In the above code ~Foo() is not called, only ~NonCopyable(), because the latter is not virtual.  Making it virtual would add overhead to all derived classes which don't already have a virtual method.

We can also change public->protected and protected->private in NonCopyable.