There are 2 possible scenarios which are not handled by the method.
- Moving content node to new <blockquote> has already been moved to outside
of the editing host.
- There is no container to insert new <blockquote>, e.g., in an inline
editing host.
In the case #1, we should ignore the ex-child node. In the case #2, we should
abort it. Note that Chrome inserts <blockquote> even if there is no proper
container. However, such behavior is disagreed in interop-2023. Therefore,
it's okay just to abort it for now.
Depends on D180781