[css3-flexbox] changing default flex value back to "0 0 auto"

Now that we no longer default the min-size in the flex-direction to being
min-content, I think we need to go back to having the default flex value
being "0 0 auto". Having a default flex-shrink value made some sense when
we wouldn't shrink below the min-content value by default, but now it means
that common uses will shrink in ways that authors don't understand.

We've already had one internal regression due to this:
https://code.google.com/p/chromium/issues/detail?id=223542. We haven't
pushed out a build with the min-size:0 change yet, so I expect we would
hear about more regressions if we don't change both of these things at the
same time. Anecdotally, I keep hearing from authors being confused by this.

This simple case, now no longer does what most people would expect:

<div style="display: -webkit-flex; width: 400px;">
    <div style="width: 400px; height: 20px;"></div>
    <div style="width: 12px; height: 20px;"></div>
</div>

The second div gets a width of 0px instead of overflowing the container due
to the flex-shrink. It's confusing and unexpected that you need to set
-webkit-flex in order to get the div to respect the explicit width that you
set.

Ojan