Move toXYZD50() to SkColorSpace_Base

SkColorSpace needs to become more versatile, in order to support
profiles that cannot specified with just a "to XYZ D50" matrix.

This a just first step to clean up the public API.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2381553002

Review-Url: https://codereview.chromium.org/2381553002
diff --git a/src/core/SkColorSpace_Base.h b/src/core/SkColorSpace_Base.h
index 9382153..07fa383 100644
--- a/src/core/SkColorSpace_Base.h
+++ b/src/core/SkColorSpace_Base.h
@@ -189,6 +189,7 @@
 
     const SkColorLookUpTable* colorLUT() const { return fColorLUT.get(); }
 
+    const SkMatrix44& toXYZD50() const { return fToXYZD50; }
     const SkMatrix44& fromXYZD50() const;
 
 private:
@@ -214,6 +215,7 @@
     sk_sp<SkGammas>           fGammas;
     sk_sp<SkData>             fProfileData;
 
+    const SkMatrix44          fToXYZD50;
     mutable SkMatrix44        fFromXYZD50;
     mutable SkOnce            fFromXYZOnce;