flex-pack:center and flex-item-align:center should do true centering

[Ojan Vafai]

http://dev.w3.org/csswg/css3-flexbox/#flex-align: "If the cross size of the flexbox is less than than that of the flexbox item, it will overflow equally in both directions."

http://dev.w3.org/csswg/css3-flexbox/#flex-pack: "If the leftover free-space is negative, the flexbox items will overflow equally in both directions."

http://plexode.com/eval3/#ht=%3Cdiv%20style%3D%22display%3A-webkit-flexbox%3B%20-webkit-flex-pack%3Acenter%3B%20border%3A%202px%20solid%20blue%3B%20width%3A%2010px%3B%22%3E%0A%20%20%20%20%3Cdiv%20style%3D%22background-color%3Ared%22%3Efoo%3C%2Fdiv%3E%0A%3C%2Fdiv%3E&ohh=1&ohj=1&jt=&ojh=1&ojj=1&ms=100&oth=0&otj=0&cex=1

[Ojan Vafai]

Created attachment 126552 [details]
Patch

[Tony Chang]

Comment on attachment 126552 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=126552&action=review

If you wanted to be thorough, you could add writing mode tests.

> Source/WebCore/ChangeLog:9
> +        Also, removed passing totalPositiveFlexibility around. We don't
> +        need to know about positive/negative flex once we run the flexing algorithm.

I remember why we had this.  It was when you could flex margins.  Maybe mention that in the ChangeLog.

[Ojan Vafai]

Committed r107446: <http://trac.webkit.org/changeset/107446>