Make SkColorSpace a public API

I've trimmed uniqueID for now, and added some comments.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1996973002

Review-Url: https://codereview.chromium.org/1996973002
diff --git a/tests/ColorSpaceTest.cpp b/tests/ColorSpaceTest.cpp
index 698c3e9..097ca09 100644
--- a/tests/ColorSpaceTest.cpp
+++ b/tests/ColorSpaceTest.cpp
@@ -8,7 +8,7 @@
 #include "Resources.h"
 #include "SkCodec.h"
 #include "SkColorSpace.h"
-#include "SkColorSpacePriv.h"
+#include "SkColorSpace_Base.h"
 #include "Test.h"
 
 #include "png.h"
@@ -21,7 +21,7 @@
                        const float red[], const float green[], const float blue[],
                        const float expectedGammas[]) {
 
-    SkGammas* gammas = space->gammas();
+    SkGammas* gammas = as_CSB(space)->gammas();
     REPORTER_ASSERT(r, almost_equal(expectedGammas[0], gammas->fRed.fValue));
     REPORTER_ASSERT(r, almost_equal(expectedGammas[1], gammas->fGreen.fValue));
     REPORTER_ASSERT(r, almost_equal(expectedGammas[2], gammas->fBlue.fValue));