[LayoutNG] Split apart CreateConstraintSpaceForChild.

Introduces PrepareChildLayout which takes now does all of the pre-layout
margin calculation, margin-collapsing, etc.

Going to try and follow up with a patch which bundles margin,
bfc_offset, etc, together instead of using the class curr_*_ variables.

BUG=635619

Review-Url: https://codereview.chromium.org/2754303003
Cr-Commit-Position: refs/heads/master@{#457953}
2 files changed