Don't allow editing to remove PseudoElements

Apparently there are cases in editing where it will attempt to remove a
pseudo element. This should never happen, but when it does we should
just ignore the request to do so since otherwise we'll crash later when
we attempt to clean up the pseudo element and realize it has no parent.

BUG=338953

Review URL: https://codereview.chromium.org/143383006

git-svn-id: svn://svn.chromium.org/blink/trunk@166985 bbb929c8-8fbe-4397-9dbb-9b2b20218538