[LayoutNG] Return MinAndMaxContentSizes by value, step 1

It's small enough that we probably don't need to worry about copying it.

ComputeMinAndMaxContentSizesSync was a small convenience wrapper that
did this; we don't need it anymore.

Step 2 will tackle NGBlockLayoutAlgorithm::ComputeMinAndMaxContentSizes

BUG=635619

Review-Url: https://codereview.chromium.org/2656693007
Cr-Commit-Position: refs/heads/master@{#446421}
5 files changed