Split SkSLLayout into .cpp/.h.

Change-Id: Id4385aacbf8b8ef53e7cc099857982af0b196689
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/598566
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
7 files changed