[LayoutNG] Refactor of HandleInflow children.

This moves a bunch of code around and some additional comments.

Before the code for handling an inflow element was a little difficult to follow.
Now most of the code and complexities are inside HandleInflow. Functions which
this calls are now relatively simple.

BUG=635619

Review-Url: https://codereview.chromium.org/2975973002
Cr-Commit-Position: refs/heads/master@{#486260}
3 files changed