Added kSRGBLinear_Named color space, along with tests

include/core/SkColorSpace.h

Index: include/core/SkColorSpace.h
diff --git a/include/core/SkColorSpace.h b/include/core/SkColorSpace.h
index b2b005e4aa6b8c2e6e82b4f36474a5daec68dea9..a6a800d140d9e0faf408cc50896dcdb0a893429c 100644
--- a/include/core/SkColorSpace.h
+++ b/include/core/SkColorSpace.h
@@ -31,6 +31,12 @@ public:
          *  Often used by images and monitors.
          */
         kAdobeRGB_Named,
+
+        /**
+         *  Colorspace with the sRGB primaries, but a linear (1.0) gamma. Commonly used for
+         *  half-float surfaces, and high precision individual colors (gradient stops, etc...)
+         */
+        kSRGBLinear_Named,
     };
 
     enum RenderTargetGamma : uint8_t {