[css3-flexbox] flex-basis initial value should be 0px

I think this was probably an oversight in the recent changes to the spec,
but at some point, the default value of flex-basis changed from 0px to auto.

"‘flex: <positive-number>’
Equivalent to ‘flex: <positive-number> 1 0px’. This value makes the flex
item flexible, and sets the flex basisto zero, resulting in an item that
receives the specified proportion of the free space in the flex container.
If all items in the flex container use this pattern, their sizes will be
proportional to the specified flex ratio."

That seems like the correct default behavior. Having auto as the preferred
size is considerably slower and often not what the developer wants. It
should not be the default value.

"flex:auto == flex:1 1 auto" and "flex:none == flex: 0 0 auto" both seem
fine to me as is, but the default value for flex should be "1 1 0px".

Ojan