[LayoutNG] Remove NGConstraintSpaceBuilder "copy" constructor.

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: Id5cdd2dd33b8d4ee655c26a3ba5f87f046506f2c
Reviewed-on: https://chromium-review.googlesource.com/617062
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495975}
26 files changed