Tests that adding a new rule can be undone.

After adding new rule:
[expanded] 
element.style { ()
    font-size: 12px;

[expanded] 
div.foo { (*inspector-stylesheet:3 -> inspector-stylesheet:3:10)

[expanded] 
div.foo { (*inspector-stylesheet:1 -> inspector-stylesheet:1:10)

[expanded] 
div { (user agent stylesheet)
    display: block;
    unicode-bidi: isolate;

===== Style sheet text: =====
div.foo {}

div.foo {}
=============================
After undo:
[expanded] 
element.style { ()
    color: red;

[expanded] 
div.foo { (*inspector-stylesheet:1 -> inspector-stylesheet:1:10)

[expanded] 
div { (user agent stylesheet)
    display: block;
    unicode-bidi: isolate;

===== Style sheet text: =====
div.foo {}
=============================
After redo:
[expanded] 
element.style { ()
    font-size: 12px;

[expanded] 
div.foo { (*inspector-stylesheet:3 -> inspector-stylesheet:3:10)

[expanded] 
div.foo { (*inspector-stylesheet:1 -> inspector-stylesheet:1:10)

[expanded] 
div { (user agent stylesheet)
    display: block;
    unicode-bidi: isolate;

===== Style sheet text: =====
div.foo {}

div.foo {}
=============================