[LayoutNG] Fix some margin issues with positioning new FCs

This fixes a couple of simple issues I noticed before (but not
everything), namely:
 - Only consider the fragment's size when searching for a layout
 - Adjust the origin_offset for searching for layout opportunities by
   the child's LineLeft margin.
 - Apply auto margins within the fitting layout opportunity.

I'm going to fix some other things in other patches. Namely, "auto"
children need to stretch to their layout opportunity, etc.

Soon we are going to need to rework the exclusion space layout opp
iterator algorithm as it doesn't appear to be handling right floats

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Ibed8feefa793472a1a6d79b727151eaaadd822be
Reviewed-on: https://chromium-review.googlesource.com/661737
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503956}
7 files changed