Replace sksg::Group::shrink_to_fit() with a factory accepting an
externally-built children array.
This provides more control for clients (e.g. reserve() instead of
shrink_to_fit()).
Change-Id: Iad587435e0e9da15251a9d3bc2510ca945950b5d
Reviewed-on: https://skia-review.googlesource.com/152861
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
cd9d074578