Add variation of GetConstantValueForVariable which handles ownership.

The new function `MakeConstantValueForVariable` behaves the same, except
it takes unique_ptrs as inputs and returns a unique_ptr as output.
We already had this behavior coded up in two places and I realized I
could use this functionality in other spots as well.

Change-Id: I2f0c43e8555a3c89bf9a1eef7030b422355ec2a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/405680
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
4 files changed