Added kSRGBLinear_Named color space, along with tests

Gradients (and other shaders) are going to end up serializing this
particular color space very frequently, so we want a shorthand way of
writing it out. I think it's also helpful to have a clearer way of
creating it (vs. NewNamed(kSRGB_Named)->makeLinearGamma()).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2377763002

Review-Url: https://codereview.chromium.org/2377763002
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 31ff9ab..0faf006 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -141,7 +141,7 @@
         }
 
         // Color space refers to fColors4f, so it's always linear gamma
-        fColorSpace = SkColorSpace::NewNamed(SkColorSpace::kSRGB_Named)->makeLinearGamma();
+        fColorSpace = SkColorSpace::NewNamed(SkColorSpace::kSRGBLinear_Named);
     } else {
         SkASSERT(desc.fColors4f && desc.fColorSpace && desc.fColorSpace->gammaIsLinear());
 
@@ -683,10 +683,9 @@
                                              SkColorSpace::NewNamed(SkColorSpace::kSRGB_Named));
                     break;
                 case GradientBitmapType::kHalfFloat:
-                    info = SkImageInfo::Make(kCache32Count, 1, kRGBA_F16_SkColorType,
-                                             kPremul_SkAlphaType,
-                                             SkColorSpace::NewNamed(SkColorSpace::kSRGB_Named)
-                                                ->makeLinearGamma());
+                    info = SkImageInfo::Make(
+                        kCache32Count, 1, kRGBA_F16_SkColorType, kPremul_SkAlphaType,
+                        SkColorSpace::NewNamed(SkColorSpace::kSRGBLinear_Named));
                     break;
                 default:
                     SkFAIL("Unexpected bitmap type");