Make CodecBench test kPremul and kUnpremul

bench/CodecBenchPriv.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef CodecBenchPriv_DEFINED
 #define CodecBenchPriv_DEFINED
 
 #include "SkImageInfo.h"
 
 inline const char* color_type_to_str(SkColorType colorType) {
     switch (colorType) {
         case kN32_SkColorType:
             return "N32";
         case kRGB_565_SkColorType:
             return "565";
         case kGray_8_SkColorType:
             return "Gray8";
         case kIndex_8_SkColorType:
             return "Index8";
         case kAlpha_8_SkColorType:
             return "Alpha8";
         default:
             return "Unknown";
     }
 }
 
+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