Update GrSwizzle to only store key and not string.

Change-Id: Ieaba188e620e5c1f1d423109e628b0ca78f3afe0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257394
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
13 files changed