Remove DISALLOW_ macro from libcef/ (see issue #3234)

Also perform related C++ cleanup:
- Use =default instead of {} for default implementations of
  constructors/destructors.
- Replace typedef with using.

libcef/browser/views/button_impl.h

@@ -25,7 +25,7 @@
 // view_impl.h for a usage overview.
 CEF_BUTTON_IMPL_T class CefButtonImpl : public CEF_VIEW_IMPL_D {
  public:
-  typedef CEF_VIEW_IMPL_D ParentClass;
+  using ParentClass = CEF_VIEW_IMPL_D;
 
   // CefButton methods. When adding new As*() methods make sure to update
   // CefViewAdapter::GetFor() in view_adapter.cc.