Back out changeset 8011f4e535fa (bug 768288) for jit-test failures

CLOSED TREE
Bug 870568. Change assertion about destroying an nsCocoaWindow while modal to a warning because the code handles that case. r=smichaud
Bug 869151. Implement GetParent for nsCocoaWindow so that nsBaseWidget::Destroy can find the parent and remove from the child list when destroying. r=smichaud