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

src/core/SkColorSpace.cpp

Index: src/core/SkColorSpace.cpp
diff --git a/src/core/SkColorSpace.cpp b/src/core/SkColorSpace.cpp
index 18b07bca289860dbcea49b4395a67d870ad85027..bf99b474a21f4c5e9da72056f07413f48bcfe26f 100644
--- a/src/core/SkColorSpace.cpp
+++ b/src/core/SkColorSpace.cpp
@@ -17,7 +17,7 @@ static bool color_space_almost_equal(float a, float b) {
 //////////////////////////////////////////////////////////////////////////////////////////////////
 
 SkColorSpace::SkColorSpace(GammaNamed gammaNamed, const SkMatrix44& toXYZD50, Named named)
-    : fGammaNamed(kNonStandard_GammaNamed)
+    : fGammaNamed(gammaNamed)
     , fToXYZD50(toXYZD50)
     , fNamed(named)
 {}