So, if you want the my_line to be the same size as the wrapper and shrink when you resize the window use this approach : 
.wrapper {
    width: 100%;
    max-width: 1200px;
    margin: 0 auto;
}
.wrapper .my_line {
    width: 100%;
}
That way wrapper will have 100% of the browser but be limited and always centered. While the my_line will inherit the full width.
So, when you resize, to less than 1200px the wrapper and my_line will shrink with the window.
EDIT:
If you want the my_line to be bigger than the wrapper, why placing it inside the wrapper ? Just place it above or underneath of the wrapper. 
The solution you got works anyways if you change the HTML. The problem with your solution is that if you later add position: relative to the wrapper it won't work. While if you change the HTML structure so that my_line is direct children of the body it will still work.