add MakeS32 helper to SkImageInfo, fix named-gamma constructor bug

include/core/SkImageInfo.h

Index: include/core/SkImageInfo.h
diff --git a/include/core/SkImageInfo.h b/include/core/SkImageInfo.h
index af3d7393b3c51b914beaaac8a9c70c25aa47a915..743ca5948137f0cd7e8d816bc4b2a3c45c84981a 100644
--- a/include/core/SkImageInfo.h
+++ b/include/core/SkImageInfo.h
@@ -223,6 +223,11 @@ public:
         return MakeN32Premul(size.width(), size.height(), pt);
     }
 
+    /**
+     *  Create an ImageInfo marked as SRGB with N32 swizzle.
+     */
+    static SkImageInfo MakeS32(int width, int height, SkAlphaType at);
+
     static SkImageInfo MakeA8(int width, int height) {
         return SkImageInfo(width, height, kAlpha_8_SkColorType, kPremul_SkAlphaType,
                            kLinear_SkColorProfileType, nullptr);