Make GrSwizzle::GrSwizzle() constexpr

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5060

Change-Id: I862b895fd92b9b6938627af7b21022564b535e9b
Reviewed-on: https://skia-review.googlesource.com/5060
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed