Default for flex-shrink when flex-grow is 0 in the shorthand

To address other issues, we changed the default for 'flex-shrink' from 0 to 1 during the F2F.

The behavior of “flex: 0 <length>;” changes - it can now shrink, where before it couldn't.

Proposal A

flex-shrink always gets reset to 1, even if flex-grow is 0.

Proposal B

Make an omitted flex-shrink default to 0 if the flex-grow is given as 0 in the flex shorthand.

This issue ties in with the initial values of flex, whose outcome may affect what's sensible here.