[css3-flexbox] Change the behavior of omitted flex-shrink in the flex shorthand

Issue link: http://wiki.csswg.org/topics/css3-flexbox-default-shrink-when-grow-is-0

We discussed this at the F2F, but didn't come to a conclusion, as it
was right at the end of the period.

To address other issues, we changed the default for 'flex-shrink' from
0 to 1 during the F2F.  However, this means that the behavior of
“flex: 0 <length>;” changes - it can now shrink, where before it
couldn't.

We propose to make an omitted flex-shrink default to 0 if the
flex-grow is given as 0 in the flex shorthand.  I think dbaron was
weakly against this during the F2F.  David, how do you feel?

~TJ