We should change CompositeEditCommand::DeleteSelection(EditingState*, ...)
to return |true| if not aborted and |false| if aborted and it
should abort if one of ending_selection_ or starting_selection_ is not
valid for |GetDocument()|.


Current pattern:
    DeleteSelection(editing_state, false, true);
    if (editing_state->IsAborted())
      return;
    // |ending_selection_| may not be valid for document.
    insertion_position = EndingVisibleSelection().Start();
    affinity = EndingVisibleSelection().Affinity();

New pattern:
    if (!DeleteSelection(editing_state, false, true))
      return;
    insertion_position = EndingVisibleSelection().Start();
    affinity = EndingVisibleSelection().Affinity();