[layoutng] Refactor min/max content contribution a bit

Provide a helper function in ng_length_utils that calls
node.ComputeMinMaxContentSizes for you.

R=eae@chromium.org,mstensho@chromium.org

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Id5e559aa1d9991f1e39d74fd84578ca2b7641883
Reviewed-on: https://chromium-review.googlesource.com/1069628
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560840}
5 files changed