Make CodecBench test kPremul and kUnpremul

bench/CodecBenchPriv.h

Index: bench/CodecBenchPriv.h
diff --git a/bench/CodecBenchPriv.h b/bench/CodecBenchPriv.h
index d8585b60060a1ab5719c5624a07d91da2a1a9c9b..318a8c6145ddc26691054f63a2cb4451e35bb67e 100644
--- a/bench/CodecBenchPriv.h
+++ b/bench/CodecBenchPriv.h
@@ -27,4 +27,18 @@ inline const char* color_type_to_str(SkColorType colorType) {
     }
 }
 
+inline const char* alpha_type_to_str(SkAlphaType alphaType) {
+    switch (alphaType) {
+        case kOpaque_SkAlphaType:
+            return "Opaque";

[msarett, 2016/01/07 18:51:59]

I could be convinced to be silent when the decode is opaque.  For example, if we
are benching jpegs, we don't need to be told over and over that they are opaque.

But I think it is useful to know when a png is opaque vs. not-opaque.  Of
course, we could still figure this out by the absence of a message.

[scroggo, 2016/01/07 19:11:20]

I would lean towards being silent.

[msarett, 2016/01/07 19:42:23]

Done.

+        case kPremul_SkAlphaType:
+            return "Premul";
+        case kUnpremul_SkAlphaType:
+            return "Unpremul";
+        default:
+            SkASSERT(false);
+            return "Unknown";
+    }
+}
+
 #endif // CodecBenchPriv_DEFINED