TextInput: set_content sets wrong edit_point

[donaldpipowitch]

In my ongoing pull request for unit test for TextInput it looks like the edit_point is wrong if set_content is used in some cases.

From #4569 (comment):

We have the String "abc\nf", make a selection of "abc", replace the content with "de" and edit_point.index will be 1. I would expect [...] 2 as the end of "de".

From https://github.com/donaldpipowitch/servo/blob/6ce72e60b70fa43f7006576a02465d64f7f8c997/components/script/textinput.rs#L492:

#[test]
fn test_textinput_set_content() {
    let mut textinput = TextInput::new(Lines::Multiple, "abc\nde\nf".into_string());
    assert_eq!(textinput.get_content().as_slice(), "abc\nde\nf");

    textinput.set_content("abc\nf".into_string());
    assert_eq!(textinput.get_content().as_slice(), "abc\nf");

    assert_eq!(textinput.edit_point.line, 0);
    assert_eq!(textinput.edit_point.index, 0);
    textinput.adjust_horizontal(3, true);
    assert_eq!(textinput.edit_point.line, 0);
    assert_eq!(textinput.edit_point.index, 3);
    textinput.set_content("de".into_string());
    assert_eq!(textinput.get_content().as_slice(), "de");
    assert_eq!(textinput.edit_point.line, 0);
    assert_eq!(textinput.edit_point.index, 1);
}