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
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