[css-flexbox] More correctly implement percentage sizing

All stretched flex items need to be considered definite per:
https://drafts.csswg.org/css-flexbox/#algo-stretch

This implements that part of the spec as described by
redoing layout of flex items that have percentage children.

BUG=341310
R=leviw@chromium.org
CC=dgrogan@chromium.org

Review URL: https://codereview.chromium.org/1875263005

Cr-Commit-Position: refs/heads/master@{#388308}
5 files changed