Tests that perform-undo-perform of the mergeable action does not crash.

Initial value
[expanded] 
element.style  { ()

[expanded] 
.container  { (perform-undo-pe…-action.html:7 -> perform-undo-perform-of-mergable-action.html:7:1)
font-weight: bold;

[expanded] 
div  { (user agent stylesheet)
display: block;


After changing property
[expanded] 
element.style  { ()

[expanded] 
.container  { (perform-undo-pe…-action.html:7 -> perform-undo-perform-of-mergable-action.html:7:1)
font-weight: normal;

[expanded] 
div  { (user agent stylesheet)
display: block;


After undo
[expanded] 
element.style  { ()

[expanded] 
.container  { (perform-undo-pe…-action.html:7 -> perform-undo-perform-of-mergable-action.html:7:1)
font-weight: bold;

[expanded] 
div  { (user agent stylesheet)
display: block;


After perform
[expanded] 
element.style  { ()

[expanded] 
.container  { (perform-undo-pe…-action.html:7 -> perform-undo-perform-of-mergable-action.html:7:1)
font-weight: normal;

[expanded] 
div  { (user agent stylesheet)
display: block;