Make CodecBench test kPremul and kUnpremul

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1568913002

Review URL: https://codereview.chromium.org/1568913002
diff --git a/bench/CodecBenchPriv.h b/bench/CodecBenchPriv.h
index d8585b6..5028573 100644
--- a/bench/CodecBenchPriv.h
+++ b/bench/CodecBenchPriv.h
@@ -27,4 +27,18 @@
     }
 }
 
+inline const char* alpha_type_to_str(SkAlphaType alphaType) {
+    switch (alphaType) {
+        case kOpaque_SkAlphaType:
+            return "";
+        case kPremul_SkAlphaType:
+            return "Premul";
+        case kUnpremul_SkAlphaType:
+            return "Unpremul";
+        default:
+            SkASSERT(false);
+            return "Unknown";
+    }
+}
+
 #endif // CodecBenchPriv_DEFINED