-webkit-justify-conten doesn't do anything. See attached. The text is the only
child of the parent <div> (flex container) and it should be centered vert/horiztonally.
Created attachment 145921 [details]
Looks like it might have been a renaming regression as well. Changing the test-case to the old names makes the text center on Chrome 21.0.1155.2 (Official Build 139341) dev.
Ah, this is because of the change in the initial value of flex. Since the initial value is 1, the text stretches to take all the space and there's no room for justify-content.
This was brought up on the spec mailing list as to why we shouldn't have flexing by default.
This works for me now on Chrome Canary. Looks like bug 88443 landing fixed this.