[LayoutNG] Remove NGConstraintSpaceBuilder "copy" constructor.

Reland of:
https://chromium-review.googlesource.com/c/chromium/src/+/617062

This removes the NGConstraintSpaceBuilder copy constructor which
took a mutable constraint space. The constructor now just picks off
the writing_mode, and icb_size, which are the two things that are
strictly required.

This is in preparation for making NGExclusionSpace immutable when
passed between layouts, via a copy. (Needed to const-ify
NGConstraintSpace in the ng_float_utils.cc file.

Bug: 635619
Change-Id: I082066d3b6ad6ebd08beba0a1ef4101a5624a298
Reviewed-on: https://chromium-review.googlesource.com/624857
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496114}
26 files changed