Add color xform support to SkWebpCodec

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2294993002

Committed: https://skia.googlesource.com/skia/+/828ed1777da74692d0c8a5834017929f5aedcc6b
Review-Url: https://codereview.chromium.org/2294993002
diff --git a/src/codec/SkCodecPriv.h b/src/codec/SkCodecPriv.h
index 1749407..9a8a43e 100644
--- a/src/codec/SkCodecPriv.h
+++ b/src/codec/SkCodecPriv.h
@@ -331,4 +331,8 @@
     return !isLegacy && (needsPremul || isF16 || srcDstNotEqual);
 }
 
+static inline SkAlphaType select_alpha_xform(SkAlphaType dstAlphaType, SkAlphaType srcAlphaType) {
+    return (kOpaque_SkAlphaType == srcAlphaType) ? kOpaque_SkAlphaType : dstAlphaType;
+}
+
 #endif // SkCodecPriv_DEFINED