[layoutng] fix incorrect vector sizing

We need to set the capacity, not the size, to children_.size() because
otherwise the following append() calls will not do the right thing.

R=eae@chromium.org, ikilpatrick@chromium.org, glebl@chromium.org
BUG=635619

Review URL: https://codereview.chromium.org/2295233002 .

Cr-Commit-Position: refs/heads/master@{#415721}
1 file changed