float components in xfermodes

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1623483002

TBR=mtklein

Review URL: https://codereview.chromium.org/1634273002
diff --git a/include/core/SkColor.h b/include/core/SkColor.h
index 099771a..461a538 100644
--- a/include/core/SkColor.h
+++ b/include/core/SkColor.h
@@ -172,11 +172,15 @@
     };
     float fVec[4];
 
-    float a() const { return fVec[3]; }
+    float a() const { return fVec[A]; }
 
     static SkPM4f FromPMColor(SkPMColor);
 
-    bool isUnit() const;
+#ifdef SK_DEBUG
+    void assertIsUnit() const;
+#else
+    void assertIsUnit() const {}
+#endif
 };
 
 /*