Stop reversing child expression in reifyMathExpression(). As far as I can tell, this ends up being identical since those are children of a sum. However, the tests rely on a specific ordering when validating the results. When parsing something like "calc(1px + 1em)", we would end up with `1em` and first child and `1px` and second child, which was confusing.
Pull request: https://github.com/WebKit/WebKit/pull/7269