[css-flexbox] Minor performance optimization

If we lay out the child due to the intrinsic main axis size,
we don't have to force-layout it again later.

R=dgrogan@chromium.org,eae@chromium.org
TEST=n/a, no change in behavior

Review-Url: https://codereview.chromium.org/2128073002
Cr-Commit-Position: refs/heads/master@{#403976}
1 file changed