Add const to SkSL layout keys.

This will allow @if statements to work properly with layout(key) values
even with control-flow analysis disabled.

Change-Id: Ib76b8c5553621815decda7fb0eefccb2c82df02b
Bug: skia:11319
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378416
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed